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Free Flip-Book Botany Class 11th & 12th by Study Innovations

Free Flip-Book Botany Class 11th & 12th by Study Innovations. 597 pages

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Contents: Page No.
Chapter 1 Cell- The Unit of Life Part 1 1-17
Chapter 2 Cell- The Unit of Life Part 2 18-36
Chapter 3 Cell- The Unit of Life Part 3 37-43
Chapter 4 Cell- The Unit of Life Part 4 44-54
Chapter 5 Biomolecules 55-68
Chapter 6 Cell Cycle and Cell Division 69-82
Chapter 7 Biological Classification Part 1 83-102
Chapter 8 Biological Classification Part 2 103-108
Chapter 9 Biological Classification Part 3 109-122

Cell- The Unit of Life Part 1

Cell as a unit of life.

(1) Cytology : (G.k. kyios = cell ; logas = study) is the branch of biology. Which comprises the study of cell
structure and function. “Cell is the structure and functional unit of all living beings”.

All living organisms are composed of repeated structural units called cells. Each cell is independent in
performing all necessary processes of life and is the least complex unit of matter which can be called living.
Robert Hooke (1665) discovered hollow cavities (empty boxes) like compartments in a very thin slice of cork (cell
wall) under his microscope. He wrote a book “Micrographia” and coined the term cellula, which was later changed
into cell. Grew and Malpighi also observed small structures in slice of plants and animals. Leeuwenhoek was the first
to see free cells. He observed bacteria, protozoa, RBCs, sperms, etc. under his microscope.

(i) Cell theory : H.J. Dutrochet (1924) a French worker gave the idea of cell theory.

The actual credit for cell theory goes to two German scientists, a Botanist M.J. Schleiden (1838) and a
Zoologist T. Schwann(1839).They gave the concept “all living organisms are composed of cell”. Schleiden and
Schwann both supported the theory of “spontaneous generation”. They also mentioned that “the new cell arises
from nucleus by budding”. Main postulates of cell theory are :

(a) Living beings are made of cells. They may be unicellular, colonial or multicellular.

(b) Cell is a mass of protoplasm having nucleus.

(c) Cells are similar in structure and metabolisms.

(d) The functions of an organism are due to activities and interactions of cells.

(ii) Exceptions to the cell theory : Viruses, viroids and prions are an exception to the cell theory as they
are obligate parasites (sub–cellular in nature). Paramecium, Rhizopus, Vaucheria are some examples, which may or
may not be exceptions to the cell theory.

(iii) Modification of cell theory : Modification of cell theory was done by Rudolf Virchow (1885). He
proposed the “law of cell lineage” which states that cell originates from pre-existing cells. i.e. (omnis cellula-e-
cellula). It is also called “cell principle” or “cell doctrine”. It states : –

(a) Life exists only in cells.

(b) Membrane bound cell organelles of the protoplasm do not survive alone or outside the protoplasm.

(c) Cells never arise de novo. The new cells are like the parent cell in all respect.

(d) All cells have similar fundamental structure and metabolic reactions.

(e) Cells display homeostasis and remain alive.

(f) Functions of an organism as a whole are the sums of the activities and interactions of its constituent cell
units. An organism can not show functions which is absent in its cells.

Chapter 1 8

Cell- The Unit of Life Part 1

(g) Genetic information is stored in DNA and expressed within the cells.

(h) DNA controls structure and working of a cell.

(iv) The cell as a self contained unit : Autonomy of a cell is believed due to presence of DNA and its
expressibility, otherwise, cell components have different shape and function. It has two positions.

(a) Autonomy in unicellular organisms : Unicellular organisms lead to a totally independent life due to
different shape, size and role of different organelles shows division of labour. All these display homeostasis.
Unicellular organisms are more active due to large surface volume ratio.

(b) Autonomy in multicellular organisms : In multicellular organisms life activities are displayed by each
of the cells independently. Multicellular organisms have one thing advantage over unicellular organisms is division
of labour.

(v) Cellular totipotency : Totipotency was suggested by Haberlandt (1902). When cells have tendency or
ability to divide and redivide the condition of the cell is called totipotent and this phenomenon is called totipotency.

(vi) Steward’s experiment : Steward showed the phenomenon of cellular totipotency in carrot culture.
Small fragments (phloem) of mature carrot roots were placed in liquid medium in special containers and growth
factors like “coconut milk” was added. The culture developed into clumps or embryoids. When these were shifted to
semisolid media, full plants were formed. The plants flowered normally and even bore the seeds.

(vii) Surface volume ratio : Metabolically active cells are small, as small cells have higher nucleocytoplasmic
ratio for better control and higher surface volume ratio for quicker exchange of materials between the cell and its
outside environment. Larger cells have lower surface volume ratio as well as lower nucleocytoplasmic ratio. Surface
volume ratio decreases by one half if cell size doubles.

Differences between plant cell and animal cell

Plant cell Animal cell
Cell wall present. Cell wall absent.
Nucleus usually lies near periphery due to vacuole. Nucleus present near the centre.
Centrosome is usually absent from higher plant cells, Usually centrosome is present that helps in formation of
except lower motile cells. spindle fibres.
Plastids are present, except fungi. Plastids are absent.
Mitochondria is generally spherical or oval in shape. Generally tubular in shape.
Single large central vacuole is present. Many vacuoles occurs, which are smaller in size.
Number of mitochondria from 200 – 2000. Number of mitochondria is approximately 1600 – 16000 in
liver cells.
Cytoplasm during cell division usually divides by cell Cytoplasm divides by furrowing or cleavage method.
plate method.
Plant cells are capable of forming all the amino acids Animal cells cannot form all the amino acids, coenzymes and
coenzymes and vitamins. vitamins.
There is no contractile vacuole. Contractile vacuole may occur to pump excess water.
Sodium chloride is toxic to plant cells. Tissue fluid containing sodium chloride bathes the animal
Plant cells are generally well over 100 µm long. Generally much smaller than 100 µm.
Spindle formed during cell division is anastral. Spindle formed during cell division are amphiastral.

Chapter 1 9

Cell- The Unit of Life Part 1

Lysosomes present in less number. Lysosomes present in more number.
Chromosomes are larger in size. Chromosomes are smaller in size.

Important Tips

• Jan swammerdam : First to see red blood cells of frog.
• Marcello Malpighi : Observed small utricles in slice of plant and animal tissue.
• N. Grew : Initiated cell concept
• Lamarck : All living beings are formed of cells.
• Corti : First to point out living substance filled inside the cell. It was called “Sarcode” by Dujardin.
• In vivo (in life) study : Study of cells in their natural environment within the intact organism.
• In vitro (cultural condition) study : Study of isolated life system in laboratory and cultural condition .
• Max Shultze proposed protoplasm theory.
• Sachs proposed organismic theory.
• Crystallo : colloidal theory (Fischer), substances dispersed and dissolved in water forming both true solution as well as colloidal

• Energy transducers : Photosynthetic cells are called energy transducers because they convert radiant energy to chemical energy and

store it as food energy.
• Intrinsic information is primary while hormonal information is extrinsic and secondary information.
• Largest organelles is nucleus. Largest cytoplasmic organelle is mitochondria in animal cells and chloroplast in plant cell.
• Smallest component is microfilament but smallest organelle is ribosome.
• Viruses do not have cellular structure.
• Monerians and protistians are not divisible into cells they are rather acellular.
• Certain organisms are multinucleated eg., Rhizopus, Vaucheria, etc.
• Fibre of ramie, Boehameria nivea longest plant cell (55 cm in size).
• The shrunken state of RBC caused by exosmosis is called crenation.
• In human beings cell of kidney are smallest and of nerve fibre largest.
• Pyrenoid is a proteinaceous body around which starch is stored in green algae.
• The smallest cell considered so far is of PPLO (Pleuropneumonia like organisms) or Mycoplasma gallisepticum i.e. 0.1 µ.

• The largest cell is an egg of ostrich.
• Acetabularia a unicellular green alga is about 10 cm in length.
• In the alga caulerpa (Siphonales) the length of cell may be up to one metre.
• The bacteriophages or viruses are still smaller in size (but cannot be considered as cells because of sub – cellular nature).

Structure of the cell .

(1) Introduction

(i) Study of cell is called cytology.

(ii) Study of metabolic aspects of cell component is called cell biology.

Chapter 1 10

Cell- The Unit of Life Part 1

(iii) Leeuwenhoek : First to see free cells called them “wild animalcules” and published a book “The secret of

(iv) Robert Hooke is known as father of cytology.

(v) C.P. Swanson is known as father of modern cytology/ cell doctrine.

(vi) A.K. Sharma is known as father of cytology in India.

(vii) Dougherty classified cells based on plan as prokaryotic and eukaryotic.

(2) Mesokaryon : Dodge gave the term ‘Mesokaryon’ for dinoflagellates. These are intermediate type of cell
organisation in dinophyceae of algae. In mesokaryotic there is present a true or eukaryotic nucleus with definite
nuclear membrane and chromosomes. Chromosomes are not well organised and basic proteins or histones are
absent. Nuclear membrane is persistent during cell division. Chromosomes are permanently attached to nuclear
membrane. They show dinomitosis e.g.- Dinophysis Heterocapsa, Dinothrix etc.

(3) Types of cell : Chatton gave the term prokaryote and eukaryote. Depending upon the nature of nucleus
cells are classified. A primitive ill defined or incipient nucleus is present in prokaryotes, where as in eukaryotes. Well
organised nucleus is present.

Differences between Prokaryotic and Eukaryotic cell

Prokaryotic cell Eukaryotic cell

It is a single membrane system. It is a double membrane system.

Cell wall surrounds the plasma membrane. Cell wall surrounds the plasma membrane in some
protists, most fungi and all plant cell. Animal cell lack it.

Cell wall composed of peptidoglycans. Strengthening It is composed of polysaccharide. Strengthening material

material is mureir. is chitin in fungi & cellulose in others plants.

Cell membrane bears respiratory enzymes. It lacks respiratory enzymes.

Cytoplasm lacks cell organelles e.g., Mitochondria, ER, Cytoplasm contains various cell organelles.
Golgi body etc.

Ribosomes are 70 S type. Ribosomes are 80 S type.

There are no streaming movements of cytoplasm. Cytoplasm show streaming movements.

Endocytosis and exocytosis do not occur. Endocytosis and exocytosis occur in animal cells.

Mitotic spindle is not formed in cell division. Mitotic spindle is formed in cell division.

The mRNA does not need processing. The mRNA needs processing.

Nuclear material is not enclosed by nuclear envelope and It is enveloped by nuclear envelope. Nucleus is distinct

lies directly in cytoplasm. It is called nucleoid. from cytoplasm.

DNA is circular and not associated with histone proteins. Nuclear DNA is linear and associated with histone proteins
extranuclear DNA is circular and protein free.

Replication of DNA occurs continuously through out cell Replication of DNA occurs during S- Phase of cell cycle

cycle. only.

These have small size (0.5 to 10 µ m ) and have much less These are relatively large (10 – 15 µ m ) and have much

DNA. more DNA.

Chapter 1 11

Cell- The Unit of Life Part 1

Sexual reproduction absent but parasexuality present. Sexual reproduction is present.

Plasmids and pili occur in many prokaryotes There are no plasmids and pili in eukaryotic cells

Example – E. coli Example – Spirogyra, Chlorella

Cell division mostly amitotic. Cell division is typically mitotic.

Plasma invaginates and from finger like process. Absent
Mesosome which take part in respiration

(4) Cell compartmentation map

Cell components

Cell wall Protoplasm

Middle lamellae Primary Secondary wall Tertiary wall Cytoplasm Nucleus

Nuclear Nucleoplasm Nucleolus Chromatin
membrane material

Ectoplasm Endoplasm
(Plasma membrane)

Cell organelles Hyloplasm (Cytosole)

Without unit With single unit With double Organic contents Inorganic contents
membrane membrane unit membrane Non metals
Reserve food material Metals
Ribosome E.R. Mitochondria Excretory products
Nucleolus Golgi body Plastid Secretory products
Centriole Lysosome Nucleus
Kinetosome etc. Glyoxysome
Microtubule etc.

Cell wall.

(1) Discovery : It was first discovered by Robert Hooke in 1665.

Cell wall is the outer most, rigid, protective, non living and supportive layer found in all the plant cells,
bacteria, cyanobacteria and some protists. It is not found in animal cells.

(2) Chemical composition : Mainly cell wall consists of two parts, matrix and cellulosic fibres (microfibriles).
Matrix consists of hemicellulose, pectin, glycoproteins, lipids and water. A cellulose molecule is long unbranched

Chapter 1 12

Cell- The Unit of Life Part 1

chain of glucose molecules. There are about 6,000 glucose units in each chain. In most of the plants cell wall is
made up of cellulose (C6H10O5)n, a polymer made-up of unbranched chain of glucose molecule linked by
β,1 − 4 glycosidic bond. About 100 molecules of cellulose form a micelle, about 20 micelle form a microfibril and

approx 200 microfibril form a fibril. The cell wall of bacteria and the inner layer of blue green algae is made-up of
mucopeptide and not of cellulose. The mucopeptide is a polymer of two amino sugars namely N-acetyl glucosamine
(NAG) and N-acetyl muramic acid (NAM) held alternately in β –1,4- linkage. In higher fungi, the cell wall is made
up of chitin, polymer of glucosamine.

Pectin is a mixture of polymerised and methylated galacturans, galacturonic acid and neutral sugars.
Hemicellulose is a mixture of polymerised xylans, mannans, glucomannans, galactans, xyloglucans and
arabinogalactans. Glycoproteins are known to influence metabolic activities of the wall. A glycoprotein called
extensin or expansin takes part in loosening and expansion of cell was through incorporation of cellulose molecules
to cellulose microfibrils.

Plant cell wall may have lignin for strength (e.g., woody tissue), silica for stiffness and protection (e.g.,
epidermal cells of grasses, Equisetum), cutin for preventing loss of water (e.g., epidermal cells), wax as component
of cuticle and surface bloom as water repellent (floating leaves) and checking transpiration, suberin for
impermeability (e.g., cork cells, endodermal cells), etc.

(3) Structure : Cell wall consists of middle lamella, primary wall, secondary wall, tertiary wall.

(i) Middle lamella : Middle Middle Lamella Middle Lamella Primary Wall
lamella is the outermost region which Primary Wall
functions as a cementing layer between Lumen Secondary
two cells. It is absent on the outer free wall Layers
surface. It ruptures to create Secondary wall
intercellular spaces. Middle lamella is Layers S1 S1
formed of calcium and magnecium S2 S2
pectate. Fruit softening is due to S3 S3
gelatinisation of pectic compounds of
middle lamella. Pectin is used as T. S. of A Plant cell L.S. cell walls of two
commercial jellying agent. Which is adjacent cells
present outside the primary wall.
Fig : Layers of cell wall in T.S. and L.S. of a cell

(ii) Primary wall : A young plant cell forms a single layer of wall material. This layer is known as the primary
cell wall. The primary wall is thin, elastic and capable of expansion in a growing cell. It grows by intussusception.
Meristematic and parenchymatous cells have primary cell wall only. The cells of leaves and fruits too have only
primary wall.

(iii) Secondary wall : In mature cell, more layers of wall material are added internal to the primary wall.
These are called the secondary cell wall. Growth by addition of new wall material on the primary wall is called
accretion. The secondary wall is thick and rigid. It usually consists of three layers, which are often named
S1, S2 and S3. It is found in collenchyma and sclerenchyma cells, xylem vesseles.

(iv) Tertiary wall : Sometimes tertiary wall is laid down on secondary wall, e.g., tracheids of gymnosperms. It
is composed of cellulose and xylan, another ploysaccharides.

Chapter 1 13

Cell- The Unit of Life Part 1

(4) Origin : A cell wall is organised at telophase stage of cell division. The plane and place of cell wall is
determined by the microtubules. Fragments of ER and vesicles of golgi body alligned at the equator, called as
phragmoplast, later which forms the cell plate. The synthesis of cellulose takes place by the help of enzyme cellulose
synthase present in the plasma membrane.

The cell plate forms the cell wall. A cell posses three phases of growth namely cell formation, cell elongation
and cell maturation. The formation of new cells occurs by mitotic activity. The cell elongation is initiated by an
increase in cell turgor. It is brought about by special proteins called expansion. They are of two types α − expansion
and β − expansion. As a result, lacunae or gaps appear in between the cellulose micelle. There are two possibilities

for the deposition of new wall material.

(i) By intussuception : As the cell wall stretches in one or more directions, new cell wall material secreted by
protoplasm gets embedded within the original wall.

(ii) By apposition : In this method new cell wall material secreted by protoplasm is deposited by definite thin
plates one after the other.

Differences between primary and secondary cell wall

Primary cell wall Secondary cell wall

Primary wall is laid inner to middle lamella Secondary wall is laid inner to primary wall.

It is formed in a growing cell. It is formed when the cells have stopped growing.

It is capable of extension. Extensibility is absent except in collenchyma cells.

It is single layered. It is three or more layered.

Cellulose content is comparatively low (5 – 20%). Cellulose content is comparatively high (20 – 90%).

Cellulose microfibrils are shorter, wavy and loosely They are longer, closely arranged straight and parallel.

Protein content up to 5%. Protein content up to 1%.

Hemicellulose content is high up to 50%. It is 25% of the total.

Lipid content up to 5 – 10%. Lipid is absent.

Primary wall is comparatively thin 1 – 5 µ m. It is comparatively thick 5 – 10 µ m

(5) Thickenings of cell wall : In many secondary EF
walls specially those of xylem the cell wall becomes hard
and thick due to the deposition of lignin. With the increasing AB CD
amount of lignin, deposition protoplasm is lost. First the
lignin is deposited in middle lamella and primary wall and Fig : Different types of secondary wall thickenings –
later on in secondary wall. Like cellulose lignin is permeable (a) annular (b) spiral (c) scalariform (d) reticulate (e) pitted-
to water and substances dissolved in it. Lignin is deposited
at specific places of the cell walls due to which xylem simple pits (f) pitted-bordered pit
tracheids and trachea take up following forms:

(i) Annular thickenings : Deposition of lignin takes
place in the form of rings on the inner surface of protoxylem

Chapter 1 14

Cell- The Unit of Life Part 1

cells. These rings are placed one above the other leaving some space in between each other.

(ii) Spiral thickenings : In these thickenings deposition of lignin takes place in the form of complete spiral
bands and are formed in tracheids and trachea of protoxylem.

(iii) Scalariform (Ladder like) thickenings : In these thickenings lignin is deposited in the form of
transverse rods of the ladder. The unthickened areas between the successive thickenings appear as elongated
transverse pits. This type of thickening is common in protoxylem.

(iv) Reticulate (Net like) thickenings : The lignin is deposited in the form of a net or reticulum. The
unthickened areas are irregular in shape. These are found in metaxylem.

(v) Pitted thickenings : These are found in metaxylem. In such thickening the whole inner wall is more or
less uniformly thickened leaving here and there some unthickened areas called pits.

(6) Pits : Secondary walls may have irregular thickenings at some places and these places are called pits. Pits
are of two types :–

(i) Simple pit : In which pit chamber is uniform in diameter.

(ii) Bordered pit : In which pit chamber is flask shaped in tracheids of gymnosperm and vessels of

Pit chamber Pit aperture Margo Simple pit Bordered pit
Pit cavity Border Torus

A. Simple pit B. Bordered pit C. Bordered pit pair D. Half bordered pit

(7) Plasmodesmata : Tangle (1879) first of all discovered them and were studied elaborately by Strasburger
(1901). A number of plasmodesmata or cytoplasmic strands are present in pit through which the cytoplasm of one
cell is in contact with another. Endoplasmic reticulum plays a role in origin of plasmodesmata.

(8) Intercellular spaces : In mature cells certain spaces or cavities are produced which are of 3 types.

(i) Schizogenous cavities : In mature cells, the cell walls separate from each other and form a cavity. e.g.,
resin canals in Pinus.

(ii) Lysogenous cavities : It is formed by the break down of cell walls e.g., Citrus oil cavities.

(iii) Schizo-lysogenous cavities : Both the above processes are involved in this cavity formtion e.g.,
protoxylem of maize.

(9) Function of cell wall : The cell wall serves many functions –

(i) It maintain shape of the cells.

Chapter 1 15

Cell- The Unit of Life Part 1

(ii) It protect the cells from mechanical injury.
(iii) It wards off the attacks of pathogens (viruses, bacteria, fungi, protozoans).

(iv) It provides mechanical support against gravity. It is due to the rigid cell walls that the aerial parts of the
plants are able to keep erect and expose their leaves to sunlight.

(v) The cell wall prevents undue expansion of the cell when water enters by osmosis to compensate for the
lack of contractile vacuole. This prevents bursting of cells.

(vi) It allows the materials to pass in and out of the cell.
(vii) Though permeable, the cell wall plays some regulatory role on the passage of materials into and out of the
(viii) Many enzymic activities associated with metabolism are known to occur in the cell wall.
(ix) Cutin and suberin deposits check loss of water form the cell surface by evaporation.

(x) The cell wall helps in the maintenance of balance of intracellular osmotic pressure with that of its

(xi) Pores in the cell walls permit plasmodesmata to link up all the protoplasts into a system called symplast

(xii) The walls of xylem vessels, tracheids and sieve tubes allow movement of materials.
(xiii) The wall in some cases has a role in defence and offence by means of spines.
(xiv) Growth of the cell wall enables the cells to enlarge in size.
(xv) Cell wall and intercellular spaces constitute a nonliving component of plant body known as apoplasm.

Important Tips

• Peptidoglycane = murein = mucopeptide is the only cell wall material of prokaryotes. It’s sugar portion consists of NAG and NAM.
• In fungi cell wall is made up of chitin (polymer of N- acetyl glucosamine). In bacteria it is composed of protein lipid polysaccharide

having N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM).
• Cell wall proteins –

HRGP –Hydroxy proline rich glycoprotein → Phloem and cambium.
PRP– Proline rich protein → Xylem, fibres, cortex.
GRP– Glycine rich protein → Xylem.

Plasma membrane.
(1) Definition : Every living cell is externally covered by a thin transparent electron microscopic, elastic

regenerative and selective permeable membrane called plasma membrane. It is quasi fluid in nature. According to
Singer and Nicolson it is “protein iceberg in a sea of lipid”. A cell wall lies external to plasmalemma in plant cells,
many monerans, some protists and fungal cells. Membranes also occur inside the cells. They are collectively called

Chapter 1 16

Cell- The Unit of Life Part 1

biomembranes. The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane
covering of the portoplast. It was replaced by the term plasmalemma or plasma membrane by Plowe (1931).

(2) Chemical composition : Proteins lipoprotein (Lipid +Protein) are the major component forming 60%
of the plasma membrane. Proteins provide mechanical strength and responsible for transportation of different
substances. Proteins also act as enzyme. Lipids account may 28%-79% depending upon the type of cell and
organism involved (in humans, myelin 79%). Because of the presence of lipids, membranes are always continuous,
unbroken structures and are deformable and their over all shape can change. The lipids of plasma membrane are of
three types namely phospholipids, glycolipids and sterols. A glycolipid may be cerebroside or ganglioside. The sterol
found in the membrane may be cholesterol (Animals), phytosterol (Plants) or ergosterol (Microorganisms). A lipid
molecule is distinguishable into a head of glycerol and two tails of fatty acids.

Carbohydrates form 2%–10%. Oligosaccharides are the main carbohydrates present in plasma membrane.
The carbohydrates of plasma membrane are covalently linked to both lipid and protein components. The common
sugars found in the plasma membrane are D – glucose, D – mannose, D – glactose, N – acetyl glucosamine, N – acetyl
galoactosamine (Both are amino sugars) and sialic acid. Generally the terminal sugar of oligosaccharides is sialic
acids (Also known as N – acetylneuraminic acid NANA) which gives them a negative charge.

(3) Ultra structure : Under electron microscope the plasma membrane appears three layered, i.e. trilaminar
or tripertite. One optically light layer is of lipid and on both sides two optically dense protein layers are present.

Generally the plasma membrane is 75 Å thick (75 – 100Å), light layer is 35 Å while dark layers are
20 Å + 20 Å in thickness.

(4) Molecular structure and different models : Several models have been proposed to explain the
structure and function of the plasma membrane.

(i) Overton’s model : It suggests that the plasma membrane is composed of a thin lipid bilayer.

(ii) Sandwich model : It was proposed by Davson and Danielli (1935). According to this model the light
biomolecular lipid layer is sandwiched between two dense protein layers. This model was also said to be unit
membrane hypothesis.

(iii) Robertson’s unit membrane model : It states that all cytoplasmic membranes have a similar structure
of three layers with and electron transparent phospholipid bilayer being sandwiched between two electron dense
layer of proteins. All biomembranes are either made of a unit membrane or a multiple of unit membrane. Its
thickness is about 75 Å with a central lipid layer of 35 Å thick and two peripheral protein layers of 20 Å thick.

(iv) Fluid mosaic model : The most important and widely accepted latest model for plasma membrane was
given by Singer and Nicolson in 1972. According to them it is “protein iceberg in a sea of lipids.”

Chapter 1 17

Cell- The Unit of Life Part 2

According to this model, the cell membrane consists of a highly viscous fluid matrix of two layers of

phospholipid molecules. These serve as relatively impermeable barrier to the passage of most water soluble

molecules. Protein molecules occur in the Boundary lipid
membrane, but not in continuous layer;

Instead, these occur as separate particles

asymmetrical arranged in a mosaic pattern. Polar end

Some of these are loosely bound at Non-polar end
the polar surfaces of lipid layers, called
peripheral or extrinsic proteins. Others Lipid

penetrate deeply into the lipid layer called

integral or intrinsic proteins. Some of the

integral proteins penetrate through the Intrinsic protein
phospholipid layers and project on both the
surface. These are called trans membrane or Lipid Hydrophobic tail Intrinsic Extrinsic
tunnel proteins (glycophorins). Singly or in Hydrophilic head protein proteins
groups, they function as channels for
passage of water ions and other solutes. Fig : Fluid-mosaic model of the plasma membrane. Proteins floating in a
The channels may have gate mechanism sea of lipid. Some proteins span the lipid bilayer, others are exposed
only to one surface or the other (Modified after De Robertis et al.; 1975).

for opening in response to specific condition. The carbohydrates occur only at the outer surface of the membrane.

Their molecules are covalently linked to the polar heads of some lipid molecules (forming glycolipids) and most of

the proteins exposed at outer surface (forming glycoproteins).

The sugar protions of glycolipids and glycoproteins are involved in recognition mechanisms :–

(a) Sugar recognition sites of two neighbouring cells may bind each other causing cell to cell adhesion. This
enables cells to orientate themselves and to form tissues.

(b) Through glycoproteins, bacteria recognise each other. e.g., female bacteria are recognised by male

(c) These provide the basis of immune response and various control system, where glycoproteins act as
antigens. Lipid and integral proteins are amphipathic in nature i.e., they have hydrophilic and hydrophobic groups
with in the same molecules. The NMR (Nuclear magnetic resonance) and ESR (Electron spin resonance) studies
showed that the membrane is dynamic. The lipid tails show flexibility. The molecule can rotate or show flip flop

Difference between protein types

Extrinsic Protein Intrinsic Protein
These are associated with surface only. These lie throughout phospholipid matrix and project on both
surfaces, also called transmembrane or tunnel protein.
They form about 30% of the total membrane protein. They form about 70% of total membrane proteins.
Example – Spectrin in red blood cells & ATPase in Example – Rhodopsin in retinal rod cells.

Chapter 2 18

Cell- The Unit of Life Part 2

(5) Membrane protein can be of following types with different functions

(i) Carrier molecules : These bind with the specific molecules into or out of the cell. This provides selective
exchange of materials. The carrier protein molecules are called “permeases” e.g., Na+ – K+ pump, Na+– sugar

(ii) Receptor molecules : The glycoproteins on the cell surface act as receptors that recognize and bind with
specific molecules.

(iii) Enzyme molecules : The inner mitochondrial membrane carrier enzyme comprising the electron
transport chain for cellular respiration.

(6) Cell membranes are fluid and dynamic due to

(i) The constituent molecules can move freely in the membrane.

(ii) The cell membranes are constantly renewed during the cells life.

(iii) They can repair minor injuries.

(iv) They expand and contract during cell movement and during change in shape.

(v) They allow interactions of cells such as recognition of self and fusion of cells.

(7) Membrane permeability : According to permeability, membranes are classified as –

(i) Permeable membrane : They allow both solvent and solute molecules or ions through them. e.g.,
cellulose wall, lignified cell walls.

(ii) Impermeable membrane : They do not allow solute and solvent molecules. e.g., heavily cutinised or
suberinised cell walls in plants.

(iii) Semi-permeable membrane : They allow solvent molecules only. e.g., membranes of colloidion,
parchment paper and copper ferrocyanide membranes.

(iv) Differentially permeable membrane : All membranes found in plants allow some solutes to pass
through them along with the solvent molecules. e.g., plasma membrane, tonoplast (vacuolar membrane) etc.

(8) Intercellular communications/modification of plasma membrane/following structures are
derived from plasma membrane

(i) Microvilli : They are fingers like evaginations of 0.1 µ m diameter, engaged in absorption. e.g., intestinal
cells, hepatic cell, mesothelial cells. The surface having microvilli is called striated border or brush border.

(ii) Lomasomes : They are plasmalemma foldings found in fungal cells.

(iii) Mesosomes : It serves as site for cellular respiration in prokaryotes.

(iv) Tight junctions : Plasma membrane of two adjacent cells are fused at a series of points with a network
of ridges or sealing strands. e.g., capillaries, brain cells collecting tubules etc.

(v) Plasmodesmata : They are protoplasmic bridges amongst plant cells, which occur in area of cell wall pits.
It was discovered and reported by Tangle and Strasburger respectively.

(vi) Desmosomes : concerned with cell adherence.

Chapter 2 19

Cell- The Unit of Life Part 2

(9) Functions

(i) They control the flow of material through them and provides passage for different substances.

(ii) It is differentially permeable, solute particles (1-15 Å) can pass through it.

(iii) It is not only provides mechenical strength but also acts as a protective layer.

(iv) Plasma membrane is responsible for the transportation of materials, molecules, ions etc.

(v) It helps in osmoregulation.

(vi) Diffusion of gases take place through plasma membrane by simple and facilitated diffusion.

(vii) Transport of ions, small polar molecules through active (energy used) and passive transport (energy not

(viii) Gases like O2 and CO2 diffuse rapidly in solutions through membranes.

(ix) Ions and small polar molecules diffuse slowly through the membranes.

(x) Some solute molecules or ions first bind with certain specific carrier or transport proteins called permeases.

(xi) Water as well as some solute molecules and ion pass through membranes pores; pores are always
bordered by channel proteins.

(xii) When diffusion takes place through channel, called simple diffusion and through carrier proteins, called
facilitated diffusion.

(10) Membrane transport : It is passage of metabolites, by-products and biochemicals across biomembrane.
Membrane transport occurs through four methods–passive, facilitated, active and bulk. Size of the particles passing
through plasmalemma is generally 1 – 15 Å.

(i) Passive transport : No energy spent. Passive transport occurs through diffusion and osmosis.

(a) Diffusion : It is movement of particles from the region of their higher concentration or electrochemical
potential to the region of their lower concentration or electrochemical potential. Electrochemical potential operates
in case of charged particles like ions. Diffusion can be observed by opening a bottle of scent or ammonia in one
corner, placing a crystal of copper sulphate in a beaker of water or a crystal of KMnO4 on a piece of gelatin. Simple
diffusion does not require carrier molecules.

Independent Diffusion : In a system having two or more diffusion substances, each individual substance
will diffuse independent of others as per gradient of its own concentration, diffusion pressure or partial pressure
form region of higher one to region of lower one.

Rate of diffusion is proportional to difference in concentration and inversely to distance between the two ends
of the system, inversely to square root of relative density of substance and density of medium, directly to
temperature and pressure.

(b) Osmosis is diffusion of water across a semipermeable membrane that occurs under the influence of an
osmotically active solution.

(c) Mechanism of passive transport : Passive transport can continue to occur if the absorbed solute is
immobilised. Cations have a tendency to passively pass from electropositive to electronegative side. While anions
can pass from electronegative to electropositive side. There are two modes of passive transports.

Chapter 2 20

Cell- The Unit of Life Part 2

Lipid matrix permeability : Lipid soluble substances pass through the cell membrane according to their
solubility and concentration gradient, e.g., triethyl citrate, ethyl alcohol, methane.

Hydrophillic membrane channels : They are narrow channels formed in the membrane by tunnel
proteins. The channels make the membrane semipermeable. Water passes inwardly or outwardly from a cell
through these channels according to osmotic gradients. CO2 and O2 also diffuse through these channels as per their
concentration gradients. Certain small ions and other small water soluble solutes may also do so.

(d) Ultrafiltration is fine filtration that occurs under pressure as from blood capillaries, epithelia and
endothelia. It is of two types : –

 Paracellular through leaky junctions or gaps in between cells.

 Transcellular through fenestrations in the cells. ‘Dialysis’ is removal of waste products and toxins from
blood by means of diffusion between blood and an isotonic dialysing solution.

(e) Facilitated transport or Facilitated diffusion : It is passage of substances along the concentration
gradient without expenditure of energy that occurs with the help of special permeating substances called permeases.
Permeases form pathways for movement of certain substances without involving any expenditure of energy. At
times certain substances are transported alongwith the ones requiring active transport. The latter phenomenon
called cotransport. Facilitated transport occurs in case of some sugars, amino acids and nucleotides.

(ii) Active transport : It occurs with the help of energy, usually against concentration gradient. For this, cell
membranes possess carriers and gated channels.

(a) Carrier particles or Proteins : They are integral protein particles which have affinity for specific solutes.
A solute particles combines with a carrier to form carrier solute complex. The latter undergoes conformational
change in such a way as to transport the solute to the inner side where it is released into cytoplasm.

(b) Gated channels : The channels are opened by either change in electrical potential or specific substances,
e.g., Calcium channels.

Active transport systems are also called pumps, e.g., H + pump, K + pump, Cl − pump, Na+ − K + pump. The

pumps operate with the help of ATP. K + − H + exchange pump occurs in guard cells. Na+ − K + exchange pump

operates across many animal membranes. For every ATP hydrolysed, three Na+ ions are passed out while two
K + ions are pumped in. Sea Gulls and Penguins operate Na+ − K + pump for excreting NaCl through their nasal

Active transport of one substance is often accompanied by permeation of other substances. The phenomenon
is called secondary active transport. It is of two main types, cotransport (e.g., glucose and some amino acids
alongwith inward pushing of excess Na+ ) and counter-transport ( Ca2+ and H + movement outwardly as excess
Na+ passes inwardly).

(iii) Bulk transport : It is transport of large quantities of micromolecules, macromolecules and food particles
through the membrane. It is accompanied by formation of transport or carrier vesicles. The latter are endocytotic
and perform bulk transport inwardly. The phenomenon is called endocytosis. Endocytosis is of two types,
pinocytosis and phagocytosis. Exocytic vesicle perform bulk transport outwardly. It is called exocytosis. Exocytosis
performs secretion, excretion and ephagy.

Chapter 2 21

Cell- The Unit of Life Part 2

(a) Pinocytosis : (Lewis, 1931). It is bulk intake of fluid, ions and molecules through development of small

endocytotic vesicles of 100 – 200 nm in diameter. ATP, Ca2+ , fibrillar protein clathrin and contractile protein actin

are required. Fluid-phase pinocytosis is also called cell drinking. It is generally nonselective. For ions and molecules
the membrane has special receptor or adsorptive sites located in small pits. They perform adsorptive pinocytosis.
After coming in contact with specific substance, the area of plasma membrane having adsorptive sites, invaginates
and forms vesicle. The vesicle separates. It is called pinosome. Pinosome may burst in cytosol, come in contact with
tonoplast and pass its contents into vacuole, form digestive vacuole with lysosome or deliver its contents to Golgi
apparatus when it is called receptosome.

(b) Phagocytosis : (Metchnikoff, 1883). It is cell eating or ingestion of large particles by living cells, e.g., white
blood corpuscles (neutrophils, monocytes), Kupffer’s cells of liver, reticular cells of spleen, histiocytes of connective
tissues, macrophages, Amoeba and some other protists, feeding cells of sponges and coelentrates. Plasma
membrane has receptors. As soon as the food particle comes in contact with the receptor site, the edges of the latter
evaginate, form a vesicle which pinches off as phagosome.

One or more lysosomes fuse with a phagosome, form digestive vacuole or food vacuole. Digestion occurs
inside the vacuole. The digested substances diffuse out, while the residual vacuole passes out, comes in contact with
plasma membrane for throwing out its contents through exocytosis or ephagy.

Important tips

• E. Grater and H. Grendel (1926) : Proposed leaflet model which states that plasma membrane is formed of bilayer sheet of phospholipids.
• Wolpers (1941) : Proposed lattice model which states lipids are distributed in a framework of proteins.
• Hilleir and Hoffman (1953) : Proposed micellar model. Plasma membrane is formed of micelles of lipid molecules.
• Sandwich model of Danielli and Davson (1935) is based on physical and chemical properties.
• Proteins of plasma membrane provide functional specificity, elasticity and mechanical support.
• The arrangement of phospholipid molecules in bilayer forms a water resistant barrier.
• Glycoproteins of plasma membrane determine antigen specificity of cell. These glycoproteins from major histocompatible complex

(MHC) which are of specific type in every individual so act as finger print of the cell.
• Negative charge of the membrane is due to N – acetyl neuraminic acid (NANA)/sialic acid.
• Lehninger described the percentage of extrinsic and intrinsic protein.
• Harmone receptor proteins of plasma membrane of target cells act as signal transduction.
• Phospholipids show asymmetric distribution in plasma membrane lacithin and sphingomycelin mainly found in outer phospholipids layer

while cephalin and phosphatidyl serine are mainly present in inner phosphalipid layer.
• Lomasomes : Infolds of plasma membrane found in fungi. These were reported by Moore and Mclean.
• Transosomes found in follicular cells of ovary of birds and have triple unit membrane. First reported by Press(1964).
• Lipid soluble substances pass through the plasma membrane move readily than the water soluble substances.
• Term biomembrane was coined by Singer and Nicolson.
• Nehar and Sakmann discovered ion-channels in plasma membrane and they were awarded Noble prize for it in 1971.
• Pinocytosis and phagocytosis do not take place in prokaryotic cells.
• Singer and Nicolson’s model differs from Robertson’s model in the arrangement of proteins.
• Plasma membrane contains ATPase enzymes.
• Plasma gel or ectoplasm are the synonyms of plasma membrane.
• The secondary structure of the integral protein buried in the lipid bilayer of a cell membrane is nature.

Chapter 2 22

Cell- The Unit of Life Part 2


(1) Definition : Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is selectively
or differentially permeable. It is considered as “Polyphasic colloidal system”.

(2) Discoveries
(i) J. Huxley defined it as “physical basis of life”.
(ii) Dujardin (1835) discovered it and called them “sarcode”.
(iii) Purkinje (1837) renamed it as “Protoplasm”.
(iv) Hugo Von Mohl (1844) gave the significance of it.
(v) Max Schultz (1861) gave the protoplasmic theory for plants.
(vi) Fischer (1894) and Hardy (1899) showed its colloidal nature.
(vii) Altman (1893) suggested protoplasm as granular.
(3) Composition : Chemically it is composed of

Water 75 – 85% Carbon 20%
Proteins 10 – 25% Oxygen 62%
Lipids 2 – 3% Hydrogen 10%
Inorganic Materials 1% Nitrogen 3%

Trace elements – 5% ( Ca, P, Cl, S, K, Na, Mg, I, Fe, etc.)

Maximum water content in protoplasm is found in hydrophytes, i.e. 95% where as minimum in seeds, spores
(dormant organs) i.e. 10 – 15%. In animals water is less (about 65%) and proteins are more (about 15%).

(4) Physical properties of protoplasm : Cyclosis movement are shown by protoplasm. These are of two types.
(i) Rotation : In one direction, either clockwise or anticlockwise e.g., Hydrilla, Vallisneria. Found only in
(ii) Circulation : Multidirectional movements around vacuole e.g. Tradescantia.
(a) It shows stimulation or irritability.
(b) Protoplasm is polyphasic. Colloidal substance or true solution because true solution act as dispersion
medium and different colloidal particles constitute dispersed phase.
(c) It shows increased surface area and adsorption.
(d) It shows sol – gel transformation.
(e) It is highly viscous.
(f) It coagulates at 60o C or above or if treated with concentrated acids or bases.
(g) It shows Brownian movements.
(h) It’s specific gravity is slightly more than 1.
(i) It’s pH is on acidic side, but different vital activities occur at neutral pH which is considered as 7, injury
decreases the pH of the cell (i.e. 5.2 – 5.5) and if it remains for a long time, the cell dies.
(j) Scattering and dispersion of light is shown by protoplasm i.e. Tyndall effect.

Chapter 2 23

Cell- The Unit of Life Part 2


The substance occur around the nucleus and inside the plasma membrane containing various organelles and
inclusions is called cytoplasm.

(1) The cytoplasm is a semisolid, jelly – like material. It consists of an aqueous, structureless ground substance
called cytoplasmic matrix or hyaloplasm or cytosol.

(2) It forms about half of the cell’s volume and about 90% of it is water.

(3) It contains ions, biomolecules, such as sugar, amino acid, nucleotide, tRNA, enzyme, vitamins, etc.

(4) The cytosol also contains storage products such as glycogen/starch, fats and proteins in colloidal state.

(5) It also forms crystallo – colloidal system.

(6) Cytomatrix is differentiated into ectoplasm or plasmagel and endoplasm or plasmasol.
(7) Cytomatrix is three dimensional structure appear like a network of fine threads and these threads are called
microfilaments (now called actin filaments or microtrabecular lattice) and it is believed to be a part of cytoskeleton.
It also contains microtubules and inter mediate cytoplasmic filaments.

(8) Hyaloplasm contains metabolically inactive products or cell inclusions called deutoplast or metaplasts.
(9) Cytoplasmic organelles are plastid, lysosome, sphaerosome, peroxisome, glyoxysomes, mitochondria,
ribosome, centrosome, flagellum or cilia etc.

(10) The movement of cytoplasm is termed as cyclosis (absent in plant cells).

(1) Definition : (Gk – mito = thread ; chondrion = granule) Mitochondria are semi autonomous having
hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem.
These are absent in all prokaryotes like bacteria and cyanobacteria. Mitochondria are also called chondriosome,
chondrioplast, plasmosomes, plastosomes and plastochondriane.

(2) Discoveries
(i) These were first observed in striated muscles (Voluntary) of insects as granules by Kolliker (1850), he called
them “sarcosomes”.

(ii) Flemming (1882) called them “fila” for thread like structure.

(iii) Altman (1890) called them “bioplast”.

(iv) C. Benda (1897) gave the term mitochondria.

(v) F. Meves (1904) observed mitochondria in plant (Nymphaea).

(vi) Michaelis (1898) demonstrated that mitochondria play a significant role in respiration.

(vii) Bensley and Hoerr (1934) isolated mitochondria from liver cells.

(viii) Seekevitz called them “Power house of the cell”.

(ix) Nass and Afzelius (1965) observed first DNA in mitochondria.

Chapter 2 24

Cell- The Unit of Life Part 2

(3) Number of mitochondria : Presence of mitochondria depends upon the metabolic activity of the cell.

Higher is the metabolic activity, higher is the number Outer membrane Intermembranous space
e.g., in germinating seeds.

(i) Minimum number of mitochondria is one in Inner membrane
Microasterias, Trypanosoma, Chlorella, Matrix

Chlamydomonas (green alga) and Micromonas.

Maximum numbers are found (up to 50,000) in giant

Amoeba called Chaos – Chaos. These are 25 in

human sperm, 300 - 400 in kidney cells and 1000 –

1600 in liver cells. Crista
Intercristaeal space F1 Particles or
(ii) Mitochondria of a cell are collectively called Inclusions Inner membrane Oxysomes
chondriome. Ribosomes

(4) Size of mitochondria : Average size is F1 Particles or
0.5–1.00 µm and length up to 1 – 10 µ m. Oxysomes

(i) Smallest sized mitochondria in yeast cells Intermembranous Matrix B
(1 µ m3 ). space
Outer membrane
(ii) Largest sized are found in oocytes of Rana Inclusions
pipiens and are 20 – 40 µ m.

(iii) A dye for staining mitochondria is Janus B – C Matrix
(5) Ultrastructure of mitochondria : F1 Particles or chamber
Mitochondrion is bounded by two unit membranes Oxysomes
separated by perimitochondrial space (60 – 80 Å). DNA
The outer membrane is specially permeable because Intratubuli space
of presence of integral proteins called porins. The
inner membrane is selective permeable. The inner D Intermembranous
membrane is folded or convoluted to form
mitochondrial crests. In animals these are called space Inner
cristae and in plants these folding are called tubuli or Inner membrane chember
microvili. Outer membrane

The matrix facing face is called ‘M’ face and face Fig : Three dimentional structure of mitochondrion.
towards perimitochondrial space is called ‘C’ face. A. From an animal cell. B. From plant cell, C. T.S.
The ‘M’ face have some small stalked particles called
oxysomes or F1 particle or elementory particle or mitechondrion, D. One tubule
Fernandez – Moran Particles. Each particle is made
up of base, stalk and head and is about 10nm in Perimitochondrial space
length. Number of oxysomes varies to 104 to 105 per
mitochondrion and chemically they are made of Outer chamber Intracristael space
Protein layer
Lipid layer F1 Particles Respiratory chain
and enzymess

Mitochondrial crest
Inner membrane

Outer membraneF1 Particles

Fig : Molecular organization of inner membrane of mitochondria

Chapter 2 25

Cell- The Unit of Life Part 2

phospholipid core and protein cortex. Oxysomes have ATPase enzyme molecule (Packer, 1967) and therefore,
responsible for ATP synthesis. These elementary particles are also called F0 – F1 particles.

In the matrix 2–6 copies of naked, double stranded DNA (circular) and ribosome of 70 S type are present. It is
rich in G-C ratio. Basic histone proteins are absent in mitochondrial DNA. The synthesis of ATP in mitochondria is
called oxidative phosphorylation, which is O2 dependent and light independent. Cristae control dark respiration. F0
particles synthesize all the enzymes required to operate Kreb’s cycle. Inner membrane contains cytochrome.

(6) Semi-autonomous nature of mitochondrion : Mitochondria contain all requirements of protein synthesis :

(i) 70 S ribosomes.

(ii) DNA molecules to form mRNA and also replicate.

(iii) ATP molecules to provide energy.

The mitochondria can form some of the required proteins but for most of proteins, these are dependent upon
nuclear DNA and cytoplasmic ribosomes, so the mitochondria are called semi-autonomous organelles.

(7) Two states of mitochondria : When ATP synthesis is low or the respiratory chain of mitochondrion is
inhibited, it is called inactive or orthodox state, and has large amount of matrix and only a few cristae. But when
mitochondria are active or condensed state, and have small amount of matrix and highly developed cristae. This
shows that the development of mitochondria depends upon the physiological activity of the cell.

(8) Chemical composition : Cohn gave the chemical composition of mitochondrion:

Proteins = 65 – 70%

Lipids = 25 – 30% (90% phospholipids and 10% cholesterol, Vit. E., etc)

2 – 5% RNA Some amount of DNA

The mitochondrial matrix has many catabolic enzymes like cytochrome oxidase and reductases, fatty acid
oxidase, transaminase, etc.

(9) Enzymes of Mitochondria

(i) Outer membrane : Monoamine oxidase, glycerophosphatase, acyltransferase, phospholipase A.

(ii) Inner membrane : Cytochrome b,c1,c,a, (cyt.b, cyt.c1, cyt.c, cyt.a, cyt.a3) NADH, dehydrogenase,
succinate dehydrogenase, ubiquinone, flavoprotein, ATPase.

(iii) Perimitochondrial space : Adenylate kinase, nucleoside diphosphokinase.

(iv) Inner matrix : Pyruvate dehydrogenase, citrate synthase, aconitase, isocitrate dehydrogenase, fumarase,
α − Ketoglutarate dehydrogenase, malate dehydrogenase.

(10) Origin : Mitochondria are self-duplicating organelles due to presence of DNA molecules so new
mitochondria are always formed by growth and division of pre-existing mitochondria by binary fission.

Difference between outer and inner membrane of mitrochondria

Outer membrane Inner membrane
It is smooth having less area. It is infolded to form cristae hence large surface area.
It is freely permeable. Semipermeable, impermeable to coenzyme A and NAD.
It consist 50% lipid and 50% protein. It consist 80% protein and 20% lipid.
Sialic acid is more (4 – 5 time). Sialic acid is less.
Near about 14% enzymes are present. Near about 60 enzymes are present.

Chapter 2 26

Cell- The Unit of Life Part 2

(11) Functions of mitochondria

(i) Mitochondria are called power house or storage batteries or ATP mills as these are sites of ATP formation.

(ii) Intermediate products of cell respiration are used in the formation of steroids, cytochromes, chlorophyll, etc.

(iii) These are also seat of some amino acid biosynthesis.

(iv) Mitochondria also regulate the calcium ion concentration inside the cell.

(v) Site of Krebs cycle and electron transport system.

(vi) Site of thermiogenesis.

(vii) Yolk nucleus (a mitochondrial cloud and golgi bodies) controls vitellogenesis.

(viii) Mitochondria of spermatid form nebenkern (middle piece) of sperm during spermiogenesis.

(ix) It is capable of producing its own DNA.

(x) Mitochondria release energy during respiration.

(xi) Mitochondria contain electron transport system.

Important Tips

• Petite character in yeast and cytoplasmic male sterility in maize are examples of mitochondrial inheritance.
• Mitochondria are believed to be bacterial endosymbionts.
• Mitochondria show a large degree of autonomy or independence in their functioning.
• Mitochondria as a place of cellular respiration were first observed by Hogeboom. Enzymes of Kreb’s cycle or TCA cycle or citric

acid cycle are present in matrix except succinic dehydrogenase which is found attached to inner mitochondrial membrane.
• With the help of phase contrast microscope mitochondria has been studied well.
• Mitochondria can be separated by centrifugation.
• Mitochondria are called as “cell inside cell” by Schiff (1982).
• Life of mitochondria is not more than 5 days.
• Mitochondria are yellowish due to riboflavin.
• 70% of total enzymes of a cell are found in mitochondria.
• Mitochondrial genome has 200 kilobase pairs.
• Mitochondria has the similarity , with bacteria as both have 70 S ribosome, circular DNA and RNA.
• Mitochondria are rich in manganese.
• It has its own electron transport system.
• Mitochondria and chloroplasts have many resemblances.
• According to endosymbiotic origin of mitochondria by Kirns Altman, mitochondria were intially a free living, aerobic bacteria which

during to the process of evolution entered an anaerobic cell and become established as mitochondria. This theory is supported by
many similarities which exist between bacteria and mitochondria.
• Lehninger discovered oxysomes.
• Percentage of mitochondrial DNA in cells is 1% of the total cellular DNA.
• Parson discovered stalkless and hollow spherical particles present on outer surface of outer mitochondrial membrane.
• When mitochondria treated with detergents like digitonin or lubral, their outer unit membrane is removed and remaining part is
called Mitoplast

• The F1 particle is made up of five types of subunits namely α, β , γ , δ and ε. of these α is heaviest and ε is lightest.

• In prokaryotic cell, plasma membrane infolding makes a structure mesosome. Which is analogous structure of mitochondria of
eukaryotic cell (both part in respiration).

Chapter 2 27

Cell- The Unit of Life Part 2


(1) Definition : Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double
membrane envelope which store or synthesize various types of organic compounds as ATP and NADPH + H+ etc.
These are largest cell organelles in plant cell.

(2) History

(i) Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).

(ii) A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von

(iii) Park and Biggins (1964) gave the concept of quantasomes.

(iv) The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter
and Stall.

(v) The term thylakoid was given by Menke (1962).

(vi) Fine structure was given by Mayer.

(3) Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and

Leucoplasts : They are colourless plastids which generally occur near the nucleus in nongreen cells and
possess internal lamellae. Grana and photosynthetic pigments are absent. They mainly store food materials and
occur in the cells not exposed to sunlight e.g., seeds underground stems, roots, tubers, rhizomes etc. These are of
three types.

(i) Amyloplast : Synthesize and store starch grains. e.g., potato tubers, wheat and rice grains.

(ii) Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils e.g. castor endosperm, tube rose, etc.

(iii) Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains.

Chromoplasts : Coloured plastids other than green are kown as chromoplasts. These are present in petals
and fruits, imparting different colours (red, yellow, orange etc) for attracting insects and animals. These also carry on

These may arise from the chloroplasts due to replacement of chlorophyll by other pigments e.g. tomato and
chillies or from leucoplasts by the development of pigments.

All colours (except green) are produced by flavins, flavenoids and cyanin. Cyanin pigment is of two types one
is anthocyanin (blue) and another is erythrocyanin (red). Anthocyanin express different colours on different pH
value. These are variously coloured e.g. in flowers. They give colour to petals and help in pollination. They are
water soluble. They are found in cell sap.

Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in
chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus, chloroplasts are changed into

Chloroplast : Discovered by Sachs and named by Schimper. They are greenish plastids which possess
photosynthetic pigments.

Chapter 2 28

Cell- The Unit of Life Part 2

(i) Number : It is variable. Number of chloroplast is 1 in Spirogyra indica, 2 in Zygnema, 16 in S.rectospora,
up to 100 in mesophyll cells. The minimum number of one chloroplast per cell is found in Ulothrix and species of

(ii) Shape : They have various shapes

Shape Example
Cup shaped Chlamydomonas sp.
Stellate shaped Zygnema.
Collar or girdle shaped Ulothrix
Spiral or ribbon shaped Spirogyra
Reticulate Oedogonium
Discoid Voucheria

(iii) Size : It ranges from 3 – 10 µ m (average 5 µ m) in diameter. The discoid chloroplast of higher plants are
4 – 10 µ m in length and 2– 4 µ m in breadth. Chloroplast of spirogyra may reach a length of 1 mm. Sciophytes

(Shade plant) have larger chloroplast.

(iv) Chemical composition :

(a) Proteins 50 – 60%,

(b) Lipids 25 – 30% ,

(c) Chlorophyll – 5- 10 %,

(d) Carotenoids (carotenes and xanthophylls) 1 –2%,

(e) DNA – 0.5%, RNA 2 – 3%,

(f) Vitamins K and E,

(g) Quinines, Mg, Fe, Co, Mn, P, etc. in traces.

(v) Ultrastructure : It is double membrane structure. Both membranes are smooth. The inner membrane is
less permeable than outer but rich in proteins especially carrier proteins. Each membrane is 90 – 100 Å thick. The
inter-membrane space is called the periplastidial space. Inner to membranes, matrix is present, which is divided into
two parts.

(a) Grana : Inner plastidial membrane of the chloroplast is invaginated to form a series of parallel

membranous sheets, called lamellae, which form a

number of oval – shaped closed sacs, called thylakoids. Frets or Lamellae
Thylakoids are structural and functional elements of
chloroplasts. These thylakoids contain all the Outer Granum
membrane Stroma

requirements of light reactions e.g., pigments like

chlorophyll, carotenoids, plastoquinone, plastocyanin, Inner Thylakoid
etc. that are involved in photosynthesis. Each thylakoid membrane
has an intrathylakoid space, called loculus (size 10-30Å)
bounded by a unit membrane. Along the inner side of Granum in L.S.

Fig : A chloroplast in section (diagrammatic)

thylakoid membrane, there are number of small rounded para-crystalline bodies, called quantasomes (a

Chapter 2 29

Cell- The Unit of Life Part 2

quantasome is the photosynthetic unit) which can trap a mole of quantum of light and can bring about
photosynthetic act. Each quantasome contains about 230 chlorophyll molecules and 50 carotenoid molecules.

In eukaryotic plant cells, a number of thylakoids are superimposed like a pile of coins to form a granum. The
number of thylakoids in a granum ranges from 10-100 (average number is 20-50). The number of grana per
chloroplast also varies widely e.g., one granum per chloroplast in Euglena while there are 40-60 grana per
chloroplast in spinach. The size of each granum varies from 0.2 – 0.6 µ m in diameter. But in blue-green algae, the

thylakoids are not organised to form granum.

Adjacent grana are interconnected by branched tubules, called stromal lamellae or Fret-channel or Fret

(b) Stroma : It is transparent, proteinaceous and watery substance. Dark reaction of photosynthesis occurs in
this portion. Stroma is almost filled with “Rubisco” (about 15% of total enzyme, protein) enzyme CO2 is accepted
by this enzyme. CO2 assimilation results in carbohydrate formation. It has 20 – 60 copies of naked circular double
stranded DNA. Each DNA copy is 40 µ in length, which can code for 125 amino acids. All plastids of a cell called as

“Plastidome” (Dangeared 1920) in stroma. Amount of DNA per chloroplast is 10–15 g. Chloroplast genome has 145
kilobase pairs. It shows semiautonomous nature and ribosomes are of 70 S type.

(vi) Pigments of chloroplast : Willsttater and Stall observed the following pigments:

(a) Chlorophyll a : C55 H72 O5 N 4 Mg (with methyl group)

(b) Chlorophyll b : C55 H70O6 N 4 Mg (with aldehyde group)

(c) Chlorophyll c : C35 H 32O5 N 4 Mg

(d) Chlorophyll d : C54 H70 O6 N 4 Mg

(e) Carotenes, Xanthophylls : Carotenoids.

Difference between Chl. a and Chl. b

Chl. a Chl. b
Absorption peak at 430, 662. It is 453, 642.
Bluish green in colour. Yellowish green.
Soluble in petroleum, ether. Soluble in methyl alcohol.
Functional group at C3 position is CH3 Functional group attached to pyrrol ring is CHO.
Present in all green plants excepts autotrophic bacteria. Present in all green plants except blue green,
brown and red algae.
In chloroplast it is 75%. It is 25%
In reflected light Chl. a shows blood red colour while in In reflected light it show dull brown colour while in
transmitted light, it shows blue green colour. transmitted light, it shows yellowish green colour.

(vii) Chlorophylls and their presence : Term by Cavantou (1818). It’s molecule has tetrapyrollic or
porphyrin head (15 Å ×15 Å ) and phytol tail (20 Å long). Mg++ is present in the centre of porphyrin head. If
chlorophyll is burnt only Mg is left.

(a) Chlorophyll b : It is found in members of chlorophyceae.

Chapter 2 30

Cell- The Unit of Life Part 2

(b) Chlorophyll c : It is found in members of phaeophyceae, bacillariophyceae.
(c) Chlorophyll d : It is found in members of rhodophyceae.
(d) Chlorophyll e : It is found in members of xanthophyceae.
(e) Phycoerythrin and phycocyanin (phycobilins) are the red and blue green pigments in rhodophyceae
and cyanophyceae respectively.
(f) Fucoxanthin (brown pigment) in phaeophyceae.
(g) Bacteriochlorophyll (C55 H74 O6 N 4 Mg) or chlorobium chlorophyll present in photosynthetic bacteria.
These pigment are red in acidic and blue in alkaline medium.
(viii) Carotenoids : These are hydrocarbons, soluble in organic solvents. These are of 2 types:
(a) Carotenes : C40 H56 derivatives of vitamin A. Carrot coloured α, β,γ carotene, lycopene, etc.

(b) Xanthophyll : C40 H56O2, yellowish in colour, fucoxanthin, violaxanthin. Molar ratio of carotene and
xanthophyll in young leaves is 2 : 1.

(ix) Plastids are interchangeable
Leucoplast  Chloroplast


(degenerate chloroplast)

The leucoplast and chloroplast are interconvertible but once they have converted into chromoplast, the reverse
can not take place. Because, chromoplasts are aged or degenerated form of chloroplast e.g. in tomato.

Young ovary (colourless) – Leucoplast

Young fruit (green) – Chloroplast

Mature fruits (red) (due to Lycopene) – Chromoplast.

In carrot leucoplast – Chromoplast (carotene) etc.

(x) Origin of chloroplast : Plastids, like the mitochondria, are self duplicating organelles. These develop
from colourless precursors, called proplastids. They are believed to be evolved from endosymbiont origination.

(4) Function of plastids

(i) It is the site of photosynthesis, (light and dark reaction).

(ii) Photolysis of water, reduction of NADP to NADPH2 take place in granum.
(iii) Photophosphorylation through cytochrome b6 f, plastocyanine and plastoguinone etc.
(iv) They store starch or factory of synthesis of sugars.

(v) Chloroplast store fat in the form of plastoglobuli.

(vi) They can be changed into chromoplasts to provide colour to many flowers and fruits for attracting animals.

(vii) They maintain the percentage of CO2 and O2 in atmosphere.

Chapter 2 31

Cell- The Unit of Life Part 2

Important Tips

• Murphy and Leech (1978) have reported the synthesis of fatty acids in the spinach chloroplast.
• Proplastids are precursor of all type of plastids.
• Capasanthin is the pigment in carotenoids found in bacteria, fungi and chilly.
• Solar energy is trapped in lamella by chlorophylls but in bacteria trapping centre is B890.
• The chloroplast with nitrogen fixing genes (nif genes) constitute nitroplast.
• Pyrenoids : A proteinaceous core around which starch is deposited mostly found in the chloroplast of algae and in some bryophytes.
• Algal classification is based on pigmentation pattern.
• Eye spot or stigma is photosensitive carotenoid pigment.
• Intact chloroplast can be separated by sugar solution (2.5 M).
• Mitochondria and plastids both have own DNA molecules which is called as Extranuclear/ Extrachromosomal DNA.
• Plastids are absent from monerans, fungi and animals these are also absent from gametes and zoospores of plants.
• Ris and Plaut (1962) reported DNA in chloroplast and was called plastidome. It forms about 0.5% of total cellular DNA and is rich in

G-C pairs.
• Plastidoribosomes : Ribosomes of plastids and are of 70S type. These were reported by Jacobson et. al. (1963)
• Thylakoid term was given by Menke (1961).
• Transducers : Structure which are involved in energy transformations e.g. mitochondria and plastids.
• Plastids are the largest cell organelles. The plastids in the order of their increasing size are

Chloroplast → Chromoplast → Elaioplast → Aleuroplast → Amyloplast

• Quantasome is formed of 160 chlorophyll a + 70 chlorophyll b molecules and 50 carotenoid molecules.
• Scattered thylakoids in the cytoplasm of cyanobacteria and photosynthesis bacteria are known as chromatophores.
• Chromatophore term was given by Schmitz.

Endoplasmic reticulum (ER).
(1) Definition : It is well developed electron microscopic network of interconnected cisternae, tubules and

vesicles present throughout the cytoplasm, especially in the endoplasm.
(2) Discovery : Garnier (1897) was first to observe the ergastoplasm in a cell. The ER was first noted by

Porter, Claude, and Fullman in 1945 as a network. It was named by Porter in 1953.
(3) Occurrence : The ER is present in almost all eukaryotic cells. A few cells such as ova, embryonic cells,

and mature RBCs, however, lack ER. It is also absent in prokaryotic cell.
In muscle cells, it is called sarcoplasmic reticulum, myeloid bodies and nissel granules are believed to be

formed from ER. ER is little develop in meristematic cells.
(4) Chemical composition : All the components of ER are lipoperoteins and trilaminar like the plasma

membrane but differ in following

(i) Thinner (50 − 60 Å) than plasma membrane.

(ii) With less cholesterol.

Chapter 2 32

Cell- The Unit of Life Part 2

(iii) With more lipids.

(iv) The lumen is filled with fluid containing 70% phospholipids lecithin and cephalin etc.

(5) Ultrastructure : The ER is made up of three components :

(i) Cisternae : These are flattened, unbranched, sac like structures. They lie in stacks (piles) parallel to one
another. They bear ribosomes. They contain glycoproteins named ribophorin-I and ribophorin-II that bind the
ribosomes. Found in protein forming cells.

(ii) Vesicles : These are oval or rounded, vacuole like elements, scattered in cytoplasm. These are also
studded with ribosomes.

(iii) Tubules : Wider, tubular, branched elements mainly present near the cell membrane. They are free from
ribosomes. These are more in lipid forming cells.



Lamellae Vesicles Tubules

Fig : Elements of Endoplasmic Reticulum

All the three structures are bound by a single unit membrane.

(6) Types of ER : Depending upon the presence of ribosomes, the ER has been categorised into two types:

(i) A smooth or Agranular endoplasmic reticulum (SER) : It consists mainly of tubules and vesicles. It
has no ribosomes associated to it. It is well developed in the muscle cells, adipose tissue cells, interstitial cells,
glycogen storing liver cells, etc. and the cells that synthesize and secrete steroids. SER also takes part in synthesis of
vitamins, carbohydrates and detoxification. Detoxification of pollutants carcinogens and drugs is carried out SER of
liver cells and mitochondria, SER is associated with storage and release of Ca2+ ions. It gives rise to spherosomes.

(ii) Rough or Granular endoplasmic reticulum (RER) : It mainly consists of cisternae. It has ribosomes
attached on its cytoplasmic surface. It is abundant in cells engaged in production and excertion of proteins, e.g.,
plasma cells, goblets cells, pancreatic acinus cells and certain liver cells. The RER is more stable than SER. The RER
is basophilic due to the presence of ribosomes. Ribosomes are attached to ER through hydrophobic interaction.

The proteins synthesised by the ER membrane bound ribosomes pass into the ER lumen, where most of the
proteins are glycosylated. For this, an oligosaccharide is always linked to the − NH2 group on side chain of an
asparagine residue. The ER lumen serves as a compartment to contain substances which must be kept separate
from cytosol. In the ER lumen, the enzymes modify the proteins.

Chapter 2 33

Cell- The Unit of Life Part 2

Differences between SER and RER

SER or smooth endoplasmic reticulum does not possesses RER possesses ribosomes attached to its membrane.
ribosomes over the surface of its membrane.
It is mainly formed of vesicles and tubules. It is mainly formed of cisternae and few tubules.
It is engaged in the synthesis of glycogen lipids and steroids. The reticulum takes part in the synthesis of proteins.
Pores are absent so that materials synthesised by SER do not RER possesses narrow pores below its ribosomes for the
pass into its channels. passage of synthesised polypeptides into ER channels.
SER is often peripheral. It may be connected with It is often internal and connected with nuclear envelope.
Ribophorins are absent. RER contains Ribophorins I and II for providing
attachment to ribosomes.
SER gives rise to sphaerosomes. It helps in the formation of lysosome through the agency
of golgi apparatus.

(7) Origin : RER is formed from nuclear membrane while SER is formed from RER by loss of ribosomes.
Rough vesicles originate only from RER after homogenisation of cell. RER breaks in small fragments (Vesicles) and
it is called microsome (This is not a cell organelle). ER constitute cytoskeleton and also help as intracellular transport
system. And it is sensitive to irritation.

(8) Functions

(i) Synthesis and secretion of specific proteins via – golgi bodies.

(ii) Formation of protein ribophorin. Which helps in attachment of ribosome.

(iii) Give rise to SER.

(iv) Provides surface for synthesis of cholesterol, steroid, ascorbic acid and visual pigments.

(v) It helps in synthesis of harmones e.g., testosterone and estrogen.

(vi) It helps in glycogenolysis in the liver cells and brings about detoxification (SER).

(vii) Gastric cells secreting zymogen have well developed SER.

(viii) ER is a component of cytoskeleton (Spread as a net) of cell and provides mechanical support and shape
to the cell.

(ix) ER acts as segregation apparatus and divides the cytoplasm into chambers. Compartmentalisation is most
necessary for cellular life.

(x) It participates in the formation of cell-plate during cytokinesis in the plant cells by the formation of

(xi) ER has many types of enzymes e.g. ATPase, reductases, dehydrogenases and phosphatases.

(9) Sacroplasmic reticulum : It is a modified SER striated muscle fibres which forms a network of
interconnected tubules in the sarcoplasm. It helps in conduction of motor nerve impulses throughout the muscle
fibre and in the removal of lactic acid so prevents muscle fatigue. It is called “ergastoplasm” in muscle and
“nisslegranules” in nerve cells.

Chapter 2 34

Cell- The Unit of Life Part 2

Important Tips

• Annullated lamellae : It was first reported by Mc Culloch (1952) in the egg of sea urchin. Formed by blebbing of outer nuclear

• Transitional ER : It is RER without ribosomes.
• Microsome : This term was used by Claude (1941). It probably refers to these fragments of ER which are associated to ribosomes.
• Sjostrand gave the term α − cytomembrane for RER.

• Veratti (1902) reported sacroplasmic recticulum in the muscle fibers.
• Nissl’s granules are the masses of RER in the cyton of neurons.
• Myeloid bodies are the masses of tubules (S0 SER) found in retinal cells and are related with photoreception.
• Total ER in the cell – 2/3 RER + 1/3 SER.
• In rapidly dividing cells endoplasmic reticulum is poorly developed.

Golgi complex.

(1) Definition : Golgi complex is made up of various membranous system e.g. cisternae, vesicles and
vacuoles. These are also called golgi bodies, golgisomes, lipochondrion, dictyosomes, Dalton complex, idiosomes or
Baker’s body. These are also called “traffic police” of the cell.

(2) Discovery : First observed by George (1867) but it’s morphological details were given by Camillo Golgi
(1898), in nerve cells of barn fowl and cat.

(3) Occurence : It is present in all eukaryotic cells. They form 2% of total cell volume. In a cell these are
found above centriole or near nucleus. In plants,
these are scattered irregularly in the cytoplasm and
called as “dictyosomes”. These are absent in bacteria
and blue green algae, RBCs, spermatozoa of
bryophytes and pteridophytes, and sieve tube cells of
phloem of angiosperm.

(4) Size and number : The size of the golgi Fig : Arrangement of membrane, tubles and vesicles in golgi complex
body varies with the metabolic state of cell and hence
it is called pleomorphic. Large in mature functional
and secretary cell e.g., germinal cells, goblet cells, but
small size in non-secretary cells. There may be
25,000 dictysomes present in rhizoidal cells of Chara.
Average number 10 – 20 per cell. Number increases
during cell division.

(5) Structure : Under transmission electron microscope the st. of golgibodies was study by Dalton and Felix
(1954), golgi body is made of 4 parts.

(i) Cisternae : Golgi apparatus is made up of stack of flat. Sac like structure called cisternae. The margins of
each cisterna are gently curved so that the entire golgi body takes on a cup like appearance. The golgi body has a
definite polarity. The cisternae at the convex end of the dictyosome comprises forming face (F. face) or cis face.
While the cisternae at the concave end comprises the maturing face (M. face) or trans face. The forming face is
located next to either the nucleus or endoplasmic reticulum. The maturing face is usually directed towards the
plasma membranes. It is the functional unit of golgi body.

(ii) Tubules : These arise due to fenestration of cisternae and it forms a complex of network.

Chapter 2 35

Cell- The Unit of Life Part 2

(iii) Secretory vesicles : These are small sized components each about 40 Å in diameter presents along
convex surface of edges of cisternae. These are smooth and coated type of vesicles. Smooth or secretory vesicles,
which have a smooth surface and contain secretions of the cell and coated vesicles, that have rough surface. They
carry materials to or from the cisternae.

(iv) Golgian vacuoles : These are spherical components each about 600 Å in diameter. These are produced
by vesiculation of saccules of cisternae. Scattered cisternae are called dictyosomes and condition is called diffused.

(6) Function

(i) The main function of golgi body is secretion, so it is large sized among the secretory cells. Secretion are
released either by exocytosis or reverse pinocytosis.

(ii) Glycosidation of lipids i.e. addition of oligosaccharides to produce glycolipids.

(iii) Glycosylation of proteins i.e. addition of carbohydrate to produce glycoproteins.

(iv) Formation of lysosomes.

(v) Golgi body forms the cell plate. During cell division by secreting hemicellulose formation of enzyme and
hormones (Thyroxine) etc.

(vi) Matrix of connective tissue is formed by golgi complex.

(vii) In oocytes of animal, golgi apparatus functions as the centre around which yolk is deposited i.e. vitellogenesis.

(viii) Membrane of the vesicles produced by golgi apparatus join in the region of cytokinesis to produce new plasmalemma.

(ix) It is also called export house of cell.

(x) Golgi body contains phospholipids, proteins, enzymes and vitamin-c.

(xi) The golgi complex gives rise to the acrosome in an animal sperm.

(7) Origin : Most accepted view is that golgi body originates from RER-that has lost its ribosomes from this
RER arise transport vesicles that contain Golgi membrane and fuse with the saccule on the forming face of Golgi
apparatus. This is why this face is called the forming face.

Important Tips

• According to Camillo Golgi “Apparato reticulare interno” (internal reticular apparatus) is Golgi body.
• Cellulose, hemicellulose and pectin are synthesized by Golgi body.
• Metal silver impregnation technique was used by Camillo Golgi.
• Sperm acrosome is made of golgi apparatus.
• The main enzyme of golgi complex are glycosyl transferase, nucleoside diphosphatase and thiamine pyrophosphatase.
• Zymogen is processed in it.
• Term “trophospongium” given by Holmgen.
• The number of golgi bodies increase during cell division. Phragmoplast is the precursor of cell plate.
• The basophilic ergastoplasm in gland cells indicate the richness of golgi bodies.
• Root cap cells are rich in golgi complex secreting mucilage, which lubricates the root tip.
• Proteins and fats are stored in vacuoles and vesicles of golgi complex.
• In fungi, unicisternal dictyosomes are found.
• Zone of exclusion : A zone of clear cytoplasm with no ribosomes, mitochondria etc. around the golgi body.
• Perner gave the term dictyosome.
• Mollenhaver and Whaley (1963): Reported polarity in golgi complex.
• GERL : Golgi-endoplasmic reticulum-lysosome system.
• GER : Golgi associated endoplasmic reticulum.

Chapter 2 36

Chapter 3 Cell- The Unit of Life Part 3


(1) Definition : Lysosomes are electron microscopic, vesicular structures of the cytoplasm, bounded by a
single membrane which are involved in intracellular digestive activities, contains hydrolytic enzymes, so called

(2) Discovery : These were first discovered by a Belgian biochemist, Christian de Duve (1995) in the liver
cells and were earlier named pericanalicular dense bodies. Terms Lysosome was given by Novikoff under the study
of electron microscope. Maltile (1964) was first to demonstrate their presence in plants, particularly in the fungus

(3) Occurrence : These are absent from the prokaryotes but are present in all eukaryotic animal cells except
mammalian RBCs. They have been recorded in fungi, euglena, cotton and pea seeds.

(4) Shape : These are generally spherical in shape but are irregular in plant root tip cells.

(5) Size : Size range is 0.2-0.8 µm while size is 0.5 µm (500 nm).

(6) Number : Lysosomes are more in those cells which are involved in intracellular digestive activities e.g.,
WBCs of blood, histiocytes of connective tissue; phagocytes of liver and spleen; osteoclasts; cells of degenerating
tissue like tail of tadpole larva etc.

(7) Ultrastructure : Under electron microscope, a lysosome is formed of two parts :

(i) Limiting membrane : It is outer, single layered, lipoproteinous and trilaminar unit membrane. It keeps a
limit on glycoproteinous digestive enzymes.

(ii) Matrix : It is inner, finely granular and highly heterogeneous group substance inside the membrane.

(8) Types : The lysosomes change the nature of their contents at different times in the same cell. This
variation is referred to as polymorphism. On the basis of their contents, four types of lysosomes are recognised.

(i) Primary Lysosomes : A newly formed lysosome contains enzymes only. It is called the primary
lysosomes. Its enzymes are probably in an inactive state.

(ii) Secondary Lysosomes : When some material to be digested enters a primary lysosome, the latter is
named the secondary lysosome, or phagolysosome or digestive vacuole, or heterophagosome. This commonly
occurs by fusion of a primary lysosome with a vacuole (pinosome or phagosome) or a secretory granule.

Plasma Endoplasmic Primary lysosome Autophagic Plasma
membrane reticulum or storage granule vacuole membrane

Secondary Digested
lysosome mitochondrion

Food particles taken Phagosome Digestive Residual Defecation or
in by endocytosis vacuole
body exocytosis or wastes

Fig : Different types of lysosomes and their origin

Chapter 3 37

Chapter 3 Cell- The Unit of Life Part 3

(iii) Tertiary lysosomes/Residual bodies : In a secondary lysosome, the enzymes digest the incoming
materials. The products of digestion pass through the lysosome membrane into the cytoplasmic matrix for use as a
source of nutrition or energy. Indigestible matter remains in the secondary lysosome. A secondary lysosome
containing indigestible matter is known as the residual body or tertiary lysosome. The latter meets the cell by
exocytosis (ephagy).

(iv) Autophagosomes /Autolysosomes /Autophagic vaculoes : A cell may digest its own organelles,
such as mitochondria, ER. This process is called autophagy or autolysis. These are formed of primary lysosomes.
The enzymes (hydrolytic) of lysosomes digest the organelles thus enclosed. Therefore, the lysosome are sometimes
called disposal units/suicidal bags.

(9) Chemical composition : Matrix of primary lysosome is formed of hydrolases, which is involved in
hydrolysis or polymeric compounds, that operate in acidic medium at pH 5, so called acid hydrolases. Upto now 50
types of enzyme have been reported to be present in latent form in different types of lysosomes. These enzymes are
synthesized on RER, transported to cisternae of golgi body where these are packed into the lysosomes. These are as

(i) Proteases e.g., cathepsin and collagenase.

(ii) Nucleases e.g., DNAse and RNAse.

(iii) Glycosidases e.g., β-galactosidase, β-glucoronidase.

(iv) Phosphatases e.g., ATPase, acid phosphatase (marker enzyme).

(v) Sulphatases e.g., for sulphate-linked organic compounds.

(vi) Esterases e.g., phospholipase, acid lipase.

(10) Origin : Lysosomes arise from the golgi complex their membrane and hydrolytic enzymes are
synthesized on the RER and are transported invesicles to the golgi complex for modification and packaging.

(11) Functions

(i) Lysosomes take part in digestion of food through phagosomes, known as intracellular digestion.

(ii) In metamorphosis of many animals certain embryonic parts are digested by it.

(iii) Obstructing structures are destroyed by lysosome.

(iv) Lysosomes perform the function of exocytosis and endocytosis.

(v) Lysosomes of sperms provide enzyme for breaking limiting membrane of egg e.g., hyaluronidase enzyme.

(vi) They cause breakdown of ageing and dead cells.

(vii) Lysosomes functions as trigger of cell division or initiate cell division by digesting repressor molecules.

(viii) Nucleases (DNAse) of lysosomes may cause gene mutations which may cause disease like leukemia or
blood cancer (partial deletion of 21st chromosome).

(ix) Sometimes residual bodies accumulate inside the cells leading to storage diseases e.g. a glycogen storage
disease called Pompe’s disease, polynephritis Hurler’s disease (deformed bones due to accumulation of

(x) Lysosomes also engulf the carcinogens.

Chapter 3 38

Chapter 3 Cell- The Unit of Life Part 3

Important Tips

• Cholesterol, cortisol and cortisone acts as a stablizers of lysosomal membrane, while absence of oxygen, X-rays UV rays and excess of
vitamin A and E act as labilizers and weaken the lysosomal membrane.

• Polymorphism in lysosomes were described by De Robertis et. al (1971).
• Lysosomes can hydrolyse all types of organic compounds except cellulose.


(1) Definition : The ribosomes are smallest known electron microscopic without membrane,
ribonucleo–protein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein

(2) Discovery : In 1943 Claude observed some basophilic bodies and named them as microsome. Palade
(1955) coined the term ribosome (form animal cell). Ribosomes in nucleoplasm were observed by Tsao and Sato
(1959). First isolated by Tissieres and Watson (1958) from E. coli. Ribosomes found in groups are termed as
polyribosomes or ergosomes (Rich and Warner 1963 observed first time polyribosomes).

(3) Occurrence : These are found in both prokaryotes as well as eukaryotes these are present only in free
form in the cytoplasm. While in the eukaryotes the ribosomes are found in two forms in the cytoplasm, free form
and bind form (bound on RER and outer nuclear membrane). These are also reported inside some cell organelles
like mitochondria and plastids respectively called mitoribosomes and plastidoribosomes.

(4) Number : The number of ribosomes depends upon the RNA contents of the cell. These are more in
plasma cells, liver cells, Nissl’s granules of nerve cells, meristematic cells and cancerous cells.

(5) Types of ribosomes : It is determined on the basis of sedimentation coefficient measured in Svedberg
unit or ‘S’ unit and their size. Velocity of sedimentation is 1 × 10−13 cm / sec/ dyne/gm.

(i) 70S ribosomes : Found in prokaryotes, mitochondria and plastid of eukaryotes. Each is about 200 – 290Å ×
170 – 210Å in size and 2.7 ×106 dalton in molecular weight.

(ii) 80S ribosomes : Found in cytoplasm of eukaryotes. Each is about 300 – 340 Å × 200 – 240 Å in size
and 4.5 – 5.0 ×106 daltons in molecular weight.

(iii) 77S, 60S and 55S ribosomes : Levine and Goodenough (1874) observed 77S ribosomes in fungal
mitochondria 60S ribosomes in animal mitochondria and 55S in mammalian mitochondria.

(6) Structure : Each ribosome is formed of two 290Å Length 30S Subunit 40S Subunit
unequal subunits, which join only at the time of protein 300-340Å Length
synthesis. In 70S and 80S ribosomes, 50S and 30S, 60S 50S Subunit 60S Subunit
and 40S are larger and smaller subunits respectively.
Larger subunits is dome shaped and attached to ER by 210Å Width 200-240Å Width
glycoproteins called “ribophorins”. It has a depression on 70S Ribosome 80S Ribosome
the flate side which leads into a channel having
elongating polypeptide chain. It has a protuberance, a Fig : 70S and 80S ribosome
ridge and a stalk. It also has 2 binding sites. Peptidyl or P
or Donor site and Amino actyl or A or Acceptor site.

Chapter 3 39

Chapter 3 Cell- The Unit of Life Part 3

These sites are for the attachment of charged tRNA molecules. Smaller subunit is oval shaped and fits as a cap on
flat side of larger subunit. It has a platform, cleft head and base. It has binding site for mRNA. Delimiting membrane
is not found in it. Ribosomes are attached to ER through hydrophobic interactions.

(7) Chemical composition : Ribosomes are chemically composed of rRNA and proteins Ribonucleo-Protein
(RNP). Lipids are altogether absent in ribosomes. Ribosomes are strongly negative binding cations and basic dyes.
70S ribosomes has 60-65% rRNA and 35-40% proteins (ratio is 1.5 : 1). rRNAs are of three types : 23S type and
5S type rRNAs in 50S and 16S type rRNA in 30S sub-units. There are about 55 types of proteins in 70S ribosome
out of which 21 proteins are found in 30S while 34 proteins are found in 50S ribosomal sub-unit and are called

80S ribosome has 45% rRNA and 55% proteins (ratio is about 1 : 1). r-RNA are of four types : 28S, 5S and
5.8S types of rRNAs in 60S and 18S type rRNA in 40S sub-units. There are about 70 types of proteins in 80S
ribosome out of which 30 proteins are found in 40S while 40 proteins are found in 60S ribosomal sub-units. The
ribosomal proteins are basic and almost surround the rRNA. Some proteins act as structural proteins while other
proteins act as enzymes e.g., peptidyl transferase of 50S (controls the interlinking of amino acids by peptide bonds).

A 1 × 10−3 (0.001 M) molar concentration of Mg ++ is needed for the structural cohesion of ribosomes i.e., for

holding the two subunits together. If this concentration is increased by ten folds, two ribosomes unite to form a
dimer. The sedimentation coefficient of dimer of 70S ribosmes is 100S and that of 80S is 120S. By decreasing the

Mg ++ conc. to normal, the dimer breaks into monomers (single ribosomes).
+Mg++ +Mg++
70S + 70S 100S , 80S + 80S 120S
Monomers –Mg++ Dimer –Mg++

If the Mg ++ concentration is decreased to 1 × 10−4 molar, the ribosomes break up into its sub-units. The 70S

ribosome breaks up into 50S and 30S sub-units. These 50S and 30S sub-units further dissociates into RNA and
protein components. Similarly, the 80S ribosomes dissociates into 60S and 40S sub-units which further breakup
into RNA and protein components.

(8) Biogenesis of ribosome :

(i) In eukaryotes the ribosomal RNAs like 18S, 5.8S and 28S are synthesized by nucleolus and 5S RNA out of
the nucleus.

(ii) In prokaryotes both rRNA and its protein are synthesized as well as assembled by cytoplasm.

(9) Polyribosomes or Polysomes : When many ribosomes (generally 6 – 8) are attached at some mRNA
strand. It is called polysome. The distance between adjacent ribosomes is of 90 nucleotides. These are functional
unit of protein synthesis.

(10) Function :

(i) Ribosomes are also called protein factory of the cell or work branch of proteins.

(ii) Free ribosomes synthesize structural proteins and bounded ribosomes synthesize proteins for transport.

(iii) Ribosomes are essential for protein synthesis.

(iv) Help in the process of photosynthesis.

(v) They are found numerously in actively synthesizing cells like liver cells, pancreas, endocrine, yeast cells and
meristematic cells.

Chapter 3 40

Chapter 3 Cell- The Unit of Life Part 3

(vi) Ribosomes also store the proteins temporarily.

(vii) These also store rRNAs, which helps in protein synthesis.

(viii) Enzyme peptidyl transferase occurs in large subunit of ribosome which helps in protein synthesis.

(ix) Newly formed polypeptide is protected from degradation by cytoplasmic enzymes in large sub-unit of
ribosomes before releasing it into RER lumen.

Important tips

• Gunter Blobel and David Sabatini of Rockfeller university proposed signal hypothesis in 1971. Both scientist has been awarded the
Nobel prize (1999) for this protein signalling.

• Ultra-structure of ribosomal subunits was given by James A. Lake (1981).
• Palade and Kuff (1966) gave the ultrastructure of ribosomes.
• Chaperons are proteins which assist in proper folding of proteins.


Microbodies are single membrane bounded small spherical or oval organelles, which take part in oxidation
reactions other than those of respiration. They can only be seen by electron microscope. Microbodies posses a
crystalline core and granules matrix. They are following types :–

(1) Sphaerosomes

(i) Discovery : These were first observed by Hanstein (1880) but discovered by Perner (1953). Term
sphaerosomes was given by Dangeard.

(ii) Occurrence : These are found in all the plant cells which involves in the synthesis and storage of lipids i.e.
endosperm and cotyledon.

(iii) Shape, size and structure : These are spherical or oval in shape about 0.5-2.5 µm in diameter. They
contain hydrolytic enzymes like protease, ribonuclease, phosphatase, esterase etc. They are bounded by a single
unit membrane.

(iv) Function : The main function of sphaerosomes is to help in lipid metabolism. These are also known as
plant lysosomes.

(2) Peroxisomes (Uricosomes)

(i) Discovery : These were first discovered by J. Rhodin (1954) in the cells of mouse kidney with the help of
electron microscope, and were called microbodies. De Duve (1965) isolated certain sac like organelles from various
types of animals and plants. These were called peroxisomes because these contain peroxide producing enzymes
(oxidases) and peroxide destroying enzymes (catalases).

(ii) Occurrence : These are found in photosynthetic cells of plants. In animals peroxisomes are abundant in
the liver and kidney cells of vertebrates. They are also found in other organs like brain, small intestine, testis and
adrenal cortex. They also occur in invertebrates and protozoans e.g., Paramecium.

(iii) Shape, size and structure : These are spherical in shape, about 1.5 µm in size. They are bounded by a
single unit membrane. They contains granular consents condensing in the centre. Their membrane is permeable to
amino acids, uric acids, etc. They contain four enzymes of H2O2 metabolism. The enzymes urate oxidase, d-amino

Chapter 3 41

Chapter 3 Cell- The Unit of Life Part 3

oxidase, α-hydroxy acid oxidase produce H 2O2 whereas the catalases plays a significant protective role because
H 2O2 is toxic for cells.

Uric acid + O2 Urate oxidase → → H 2O2 Methyl alcohol + H 2O2 Catalase →  → H2O
Amino acid + O2 D−amino oxidase → Formic acid + H 2O2 Catalase →

(iv) Function : These are involved in the formation and degrading of H 2O2 . Plant peroxisomes are also
involved in photorespiration. In which glycolic acid oxidase enzyme oxidises the glycolic acid to glyoxylic acid. In
case of plants peroxisomes is also known as glyoxisomes.

(3) Glyoxysomes

(i) Discovery : These were discovered by Beevers in 1961 and Briedenbach in 1967.

(ii) Occurrence : These are found in fungi, some protists and germinating seeds especially in germinating
fatty seeds where insoluble lipid food reserves must be turned into soluble sugars.

Animals cannot execute this conversion because they do not posses glyoxylate enzymes.

(iii) Shape, size and structure : These are spherical in shape, about 0.5-1 µm in size, they contain enzymes
of metabolism of glycolic acid via glyoxylate cycle and bounded by a unit membrane. These are also contain

enzymes for β-oxidation of fatty acids.

(iv) Functions : The main function of glyoxysomes is conversion of fats into carbohydrates.

(4) Lomasomes : These are sac like structures found between cell wall and plasmalemma in the haustoria of
fungal hyphae. These were first discovered by Bowen and Berlin. Webster called them border bodies.


(1) Discovery : Centrosome was first discovered by Van Benden (1887) and structure was given by T. Boveri.

(2) Occurrence : It is found in all the animal cell except mature mammalian RBC’s. It is also found in most of
protists and motile plant cells like antherozoids of ferns, zoospores of algae and motile algal forms e.g.,
Chlamydomonas but is absent in prokaryotes, fungi, gymnosperms and angiosperms.

(3) Structure : Centrosome is without unit Cart-wheel structure 13 Globular subunits
membrane structure. It is formed of two darkly central rod (HUB) 250Å
stained granules called centrioles, which are
collectively called diplosome. These centrioles are 9-Spokes
surrounded by a transparent cytoplasmic area
called centrosphere or Kinetoplasm. Centriole and 3 Subtubules (Subfibres)
centrosphere are collectively called centrosome. B
Before the cell division the centrioles at each pole
of the spindle. The two centrioles are situated at A-C Connective (DM)
3 Subtubules (Subfibres)
90o to each other. Each centriole is a
microtubular structure and is formed of Globular
microtubules arranged in 9 + 0 manner (all the 9 subunits
microtubules are peripheral in position).

A 250Å

Fig : (A) T.S. Centriole (B) Three subtubules (C) A subtubule

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Chapter 3 Cell- The Unit of Life Part 3

Each microtubule is a triplet and is formed of three subtubules which are called A, B and C. A subtubule is
about 45Å thick and is formed of 13 parallel protofilaments while each of B and C subtubule is formed of 10
parallel protofilaments. Each protofilament is formed of a row of α, β-tubulin dimers. C sub-tubule of each
microtubule is linked to A sub-tubule of adjacent microtubule by a dense material (DM) strand called A-C linker, so
all the microtubules are tilted at 40o . Each microtubule is about 250Å in diameter.

Inside the microtubules, there is an intra-centriolar or cart-wheel structure which is formed of a central hub
(about 25Å in diameter) and 9 radial spokes or radial fibres. Each radial spoke ends into a dense material (DM)
thickening, called X-body or foot which is further linked to A-subtubule. Between two adjacent X-bodies there is
another DM-thickening, called Y-body, which is linked to X-body on either side and to A-C linker on outer side.

Centriole is rich in tubulin and ATPase. Centriole can replicate but has no DNA. Centrioles replicate in G2
phase of interphase of cell cycle but do not initiate cell division.

(4) Chemical composition : Centrosome is lipoproteinaceous structure. The microtubules of centriole are
composed of protein tubulin and some lipids. They are rich in ATPase enzyme.

(5) Origin : The daughter centriole is formed from the pre-existing centriole in G2 of interphase so called self-
replicating organelle.

(6) Functions

(i) The centrioles help organising the spindle fibres and astral rays during cell division. Therefore, they are
called microtubules organising centres. The cells of higher plants lack centrioles and still form a spindle.

(ii) They provide basal bodies which give rise to cilia and flagella.

(iii) The distal centriole of a spermatozoan give rise to the axial filament of the tail.

Important Tips

• Centriole is also called microcentrum or cell centre.
• Each centriole is formed of 9 × 3 = 27 subtubules or subfibres.

Cilia and flagella.

(1) Discovery : Flagellum presence was first reported by Englemann (1868). Jansen (1887) was first scientist
to report the structure of sperm flagellum.

(2) Definition : Cilia and flagella are microscopic, hair or thread-like motile structures present extra-cellularly
but originate intra-cellularly from the basal body and help in movements, locomotion, feeding, circulation etc.

(3) Occurrence : Cilia are found in all the ciliate protozoans e.g., Paramecium, Vorticella etc. flame cells of
flat worms; in some larval forms e.g., Trochophore larva of Nereis, Bipinnaria larva of starfish etc.; in some body
structures e.g. wind-pipe, fallopian tubes, kidney-nephrons etc.

Flagella are found in all the flagellate protozoans e.g., Euglena, Trichonympha etc., collar cells of sponges;
gastrodermal cells of coelenterates; spermatozoa of animals and lower plants; zoospores of algae etc. These are
absent in red algae, blue-green algae, angiosperms, nematodes, arthropodes etc.

Chapter 3 43

Chapter 4 Cell- The Unit of Life Part 4

(4) Flagella are 1 – 4 per cell where as cilia are infinity in number.

(5) Cilia are smaller and flagella are longer in size, 5 – 10 µ m and 150 µ m respectively.

(6) Structure : Both cilia flagella are structurally similar and possess similar parts-basal body, rootlets, basal
plate and shaft

(i) Basal body : These are also termed as blepharoplast Central Membrane
(kinetosome) or basal granule. It is present below the plasma Oumteicr rotubule Link
membrane in cytoplasm. The structure is similar to centriole made of microtubules Central
9 triplets of microtubules. Out of the 3 fibrils of a triplet first is A sheath
which is round and other two B and C are semi-circular. 9 triplets are Inner arm
connected to the centre by spokes. ‘C’ fibrils disappears as it enters Bridge
into shaft. Outer arm

(ii) Rootlets : Made of microfilament and providing support to Spoke Head
the basal body. These are striated fibrillar outgrowths. Interdoublet
Sublitnukbule A
(iii) Basal plate : Central fibril develop in this area. It is highly
dense and lie above plasma-membrane. Subtubule B

Radial spoke

Fig : A diagram of T.S. Cilium or flagellum

(iv) Shaft : It is the hair like projecting part of cilia and flagella which remains outside the cytoplasm. It has 9
duplets of microtubules in radial symmetry. These are called axonema. Each axonema has 11 fibrils, 9 in the
periphery and 2 in the centre. The arrangement is called 9 + 2 pattern. Central fibrils are singlet fibrils and covered
by a central sheath. 9 pheripheral fibrils are duplet and are present at 10o difference from each other. Inner fibril of
duplet is known as subfibre A with two bent arms and the outer one is subfibre-B. Peripheral fibrils are linked with
each other by peripheral linkage and with the central fibril by radial linkage.

(7) Chemical composition : Chemically, the central tubules are formed of dynein protein while the
peripheral microtubules are formed of tubulin protein. Dynein is the ATPase enzyme which hydrolyses the ATP to
provide free energy for ciliary /flagellar beating. The interdoublet linkers are formed of nexin protein. Quantitatively,
it is formed of

Proteins = 74 – 84% Lipids = 13 – 23%

Carbohydrates = 1 – 6% Nucleotides = 0.2 – 0.4%

(8) Type of flagella : There are two types of flagella.

(i) Tinsel – type : In this, flagellum has lateral hair-like processes, called
flimmers or mastigonemes.

(ii) Whiplash – type : In this, flagellum has no flimmers.

(9) Motion : Cilia beat in coordinated rhythm either simultaneously Whiplash Tinsel
(synchronus) or one after the other (metachronic rhythm). The cilia produce a
sweeping or pendular stroke. The flagella beat independently, hence produce Fig : Types of flagella
undulatory motion.

Chapter 4 44

Chapter 4 Cell- The Unit of Life Part 4

(10) Function

(i) They help in locomotion, respiration, cleaning, circulation, feeding, etc.

(ii) Being protoplasmic structure they can function as sensory organs.

(iii) They show sensitivity to changes in light, temperature and contact.

(iv) Ciliated larvae take part in dispersal of the species.

(v) The cilia of respiratory tract remove solid particles from it. Long term smoking damages the ciliated
epithelium, allowing dust and smoke particles to enter the long alveoli.

(vi) The cilia of urinary and genital tracts drive out urine and gametes.

Difference between cilia and flagella

Characters Cilia Flagella
Number More in number (may be upto 14,000 per cell). Less in number (1-8).
Size Small sized (5-10 µ m ). Large sized (upto 100-200 µ m ).

Distribution Generally distributed on whole body. Generally located at anterior end
of body.
Beating Beat in either metachronous or synchronous Beat independently.
Type of motion Sweeping or rowing motion. Undulatory motion.
Function Locomotion, feeding, circulation, etc. Only locomotion.

Important Tips

• Kinocilia : True or motile cilia e.g. of epithelial cells of respiratory tracct.
• Stereo cilia : Immobile cilia e.g. of epididymis.
• Bacterial flagellum consists of a single fibril composed of flagellin protein.


In eukaryotic cell, a framework of fibrous protein elements became necessary to support the extensive system
of membranes. These elements collectively form cytoskeleton of the cell. There are of three types.

(1) Microtubules :

(i) Discovery : These were first discovered by De Robertis and Franchi (1953) in the axons of medullated
nerve fibres and were named neurotubules.

(ii) Position : The microtubules are electron-microscopic structures found only in the eukaryotic cellular
structures like cilia, flagella, centriole, basal-body, astral fibres, spindle fibres, sperms tail, neuraxis of nerve fibres
etc. These are absent from amoebae, slime-moulds and prokaryotes.

(iii) Structure : A microtubule is a hollow cylindrical structure of about 250 Å in diameter with about 150 Å
luman. Its wall is about 50Å thick. Its walls is formed of 13 parallel, proto-tubules, each being formed of a liner
series of globular dimeric protein molecules.

(iv) Chemical composition : These are mainly formed of tubulin protein. A tubulin protein is formed of 2
sub-units : α − tubulin molecule and β − tubulin molecule which are alternatively in a helical manner.

Chapter 4 45

Chapter 4 Cell- The Unit of Life Part 4

(v) Function
(a) These form a part of cytoskeleton and help in cell-shape and mechanical support.
(b) The microtubules of cilia and flagella help in locomotion and feeding.
(c) The microtubules of asters and spindle fibres of the mitotic apparatus help in the movement of
chromosomes towards the opposite poles in cell-division.
(d) These help in distribution of pigment in the chromatophores, so help in skin colouration.
(e) These also form micro-circulatory system of the cell which helps in intracellular transport.
(f) These control the orientation of cellulose microfibrils of the cell wall of plants.
(2) Microfilament
(i) Position : These are electron-microscopic, long, narrow, cylindrical, non-contractile and proteins structures
found only in the eukaryotic cytoplasm. These are present in the microvilli, muscle fibres (called myofilaments) etc.
But these are absent from the prokaryotes. These are also associated with the pseudopodia, plasma membrane of
fibroblats, etc. These are either scattered or organized into network or parallel arrays in the cytoplasmic matrix.
(ii) Discovery : These were discovered by Paleviz et. al. (1974).
(iii) Structure : Each microfilament is a solid filament of 50-60 Å diameter and is formed of a helical series of
globular protein molecules. These are generally grouped to form bundles.
(iv) Chemical composition : These are mainly formed of actin-protein.
(v) Functions
(a) The microfilaments forms a part of cytoskeleton to support the relatively fluid matrix.
(b) The microfilaments bring about directed movements of particles and organelles along them in the cell.
(c) The microfilaments also produce streaming movements of cytoplasm.
(d) The microfilaments also cause cleavage of animal cells which is brought about by contraction of a ring of
(e) The microfilaments also participate in gliding amoeboid motion shown by amoebae, leucocytes and
(f) The microfilaments are also resoponsible for the change in cell shape curing development, motility and
(g) Myofilaments bring about muscle contraction.
(h) The microfilaments cause movements of villi to quicken absorption of food.
(i) The microfilaments are responsible for the movement of cell membrane during endocytosis and exocytosis.
(j) The microfilaments cause plasma membrane undulations that enable the firoblasts to move.
(3) Intermediate filaments
(i) Location : They are supportive elements in the cytoplasm of the eukaryotic cells, except the plant cells.
They are missing in mammalian RBCes and in the prokaryotes.
(ii) Structure : The IFs are somewhat larger than the microfilaments and are about 10 nm thick. They are
solid, unbranched and composed of nonmotile structural proteins, such as keratin, desmine, vimentin.

Chapter 4 46

Chapter 4 Cell- The Unit of Life Part 4

(iii) Functions

(a) They form a part of cytoskeleton that supports the fuild cytosol and maintains the shape of the cell.

(b) They stabilize the epithelia by binding to the spot desmosomes.

(c) They form major structural proteins of skin and hair.

(d) They integrate the muscle cell components into a functional unit.

(e) They provided strength to the axons.

(f) They keep nucleus and other organelles in place.

Differences between microtubules and microfilaments

Microtubules Microfilaments
Are hollow cylinders. Are solid rods.
About 200 to 270 Å thick. About 50 to 60 Å thick.
Composed of 13 longitudinal protofilaments each. Not composed of protofilaments.
Formed of protein tubulin. Formed of proteins actin and myosin.
Subunits are dimers that have bound GTP and GDP. Subunits are monomers that have bound ATP and ADP.
Are noncontractile. Are contractile.
Have no role in cytoplasmic streaming, endocytosis Play a role in cytoplasmic streaming, endocytosis and
and exocytosis. exocytosis.

Important Tips

• Microtubule term was given by Slautterback.

• Tubulin proteins is dimeric protein formed of two globular polypeptides called α − tubulin and β − tubulin.

• Microtubules associated proteins like Tau- protein and kinase control polymerization of tubulin dimer’s.
• Hyman (1917) proposed sol-gel-theory for amoeboid locomotion and was supported by Mast.

Metabolically inactive cell inclusions/Deutoplasmic substances/Ergastic material .

Within the cytoplasm of a cell there occur many different kinds of non-living structures which are called
inclusions or ergastic substances. They are formed as a result of metabolic activities. They are of following types:

(1) Vacuoles : It is a non-living reservoir, bounded by a differentially or selectively permeable membrane, the
tonoplast. The structure of tonoplast is similar to that of single unit membrane i.e. tripartite structure. The vacuole is
filled with cell sap or tonoplasm. The thin layer of protoplasm, pushed towards the wall of the cell is called as
primordial utricle. They contain water and minerals.

The vacuole in plants was discovered by Spallanzani. The vacuole is not air filled cavity, rather it is filled with
a highly concentrated solution the vacuolar sap. It is generally neutral, but at maturity it becomes acidic. The cell
sap contains following.

(i) Gases : CO2, O2 and N2 .

(ii) Inorganic salts : Nitrates, chlorides, sulphates, phosphates of K, Na, Ca and Mg.

(iii) Organic acids : Malic acid, formic acid, acetic acid, oxalic acid or their salts.

Chapter 4 47

Chapter 4 Cell- The Unit of Life Part 4

(iv) Sugars : Cane sugar, glucose and maltose.
(v) Soluble proteins : Enzymes.

(vi) Glycosides : Like anthocyanins (water soluble pigment)

Some protozoans have contractile vacuoles which enlarge by accumulation of fluid or collapse by expelling
them from the cell. The vacuoles may be sap vacuoles, contractile vacuoles or gas vacuoles (pseudo vacuoles).

(vii) Function of vacuoles : Vacuole maintains osmotic relation of cell which is helpful in absorption of
water. They also act as reservoir of cells. Turgidity and flaccid stages of a cell are due to the concentrations of sap in
the vacuole. In animal cell, it is phagocytic, food vacuole, autophagic or contractile in nature.

(2) Reserve food material
The reserve food material may be classified as follows :–


Nitrogenous Non-nitrogenous

Proteins Amides
Fats and oils

Starch Cellulose Sugar Glycogen Inulin

(i) Carbohydrates : Non-nitrogenous, soluble or non- soluble important reserve food material. Starch
cellulose and glycogen are all insoluble.

(a) Starch : Found in plants in the form of minute solid grains. Starch grains are of two types:

Assimilation starch : It is formed as a result of photosynthesis of chloroplasts. Diastase enzyme converts it
into soluble sugar at night time. The conversion of sugar into reserve or storage starch is brought about by
leucoplast as amyloplast.

Reserve starch : Thick layers are deposited around an organic centre called hilum. When hilum is situated
just at the centre of starch grain, it is said to be concentric e.g. pea, bean, wheat etc. While it is situated not at the
centre, but nearer the margin it is said to eccentric e.g. potato.

(b) Glycogen : Glycogen or animal starch occurs only in colourless plants like fungi. It occurs in the cytoplasm
as an amorphous body.

(c) Inulin : It is a complex type of polysaccharide, soluble found dissolved in cell sap of roots of Dahlia,
Jaruslem, Artichoke, Dandelion and members of compositae. When these roots are preserved in alcohol it
precipitates in the form of “ Sphaerites” or fan shaped crystals.

(d) Sugars : A number of sugars are found in solution of cell sap. These include glucose, fructose, sucrose,
etc. Glucose and fructose are monosaccharides while can sugar is disaccharide and occurs in beet root and sugar-

(e) Cellulose : Chemical formula is (C6 H10O5 )n . The cell wall is made up of cellulose. It is insoluble in water.

Chapter 4 48

Chapter 4 Cell- The Unit of Life Part 4

(ii) Fats and Oils : These are important reserve food material. These are always decomposed into glycerol
and fatty acids by enzymatic action. Fat is usually abundant in cotyledons than in the endosperm. e.g. flax seed
produce linseed oil, castor produce castor oil, cotton seeds produce cottonseed oil, etc.

(iii) Proteins and Amides (Aleurone grains) : Storage organ usually contain protein in the form of
crystalline bodies known as crystalloids (potato). Proteins may be in the form of aleurone grains as in pea, maize,
castor, wheat, etc. Each aleurone grain consists of a large crystalline grain of protein known as crystalloid associated
with it there is a smaller body globoid. It is not a protein but double phosphate of calcium and magnesium.

(3) Excretory Products : The organic waste products of plants are by-product of metabolism. They are
stored as inclusions. Depending upon chemical composition they are classified as:

(i) Resins : They are believed to be aromatic compounds consisting of carbon, hydrogen and oxygen and are
acidic in nature. Sometimes they are found in combination with gums and are called gum resin. e.g. Asafoetida
(heeng). These are used in making varnishes and gums.

(ii) Tannins : They are complex nitrogenous compounds of acid nature having an astringent taste. They are
used in conversion of hide into leather. With ferric salt they are largely used manufacture of ink. Presence of tannin
in plants makes its wood hard durable and germ proof.

(iii) Alkaloids : These are organic, basic, nitrogenous substance. They occur in combination with organic
acids and most of them are poisonous. From plants, cocaine, hyoscine, morphine, nicotine, quinine, atropine,
strychnine and daturine etc. are extracted.

(iv) Glucosides : Some glucosides or glycosides function as storage substance e.g. amygdaline of the bitter
almond. Erythrocyanins and Anthocyanins are responsible red and blue colour and flavines for cream colour.
Carotene is an unsaturated fatty acid and not a glycoside, gives red and orange colour to roots.

(v) Etherial and Essential oils : These consist mixture of various hydrocarbons known as tarpenes and
their oxygen derivatives. They are responsible for flavor of many fruits and scent of many flowers etc. They are
volatile and are soluble in water, ether, petroleum etc. e.g. lavender, mint, clove oil, eucalyptus oil, theme oil etc.

(vi) Mineral matter : Many minerals are waste products in plants.

(a) Calcium oxalate : It occurs in the form of crystals of various shapes.

Raphides : Needle shaped crystals are known as raphides. They are found single or in bundles. e.g. in plants
like jamikand, Colocasia, water hyacinth (Jal kumbhi), amorphophallus and aroids.

Rosette or Sphaeraphides : Star shaped crystals. They occur in special mucilaginous parenchyma cells of
the petiole of arum, water hyacinth, etc. Crystals in the form of cubes are found in tunic of onion bulb. In the leaf of
belladona, these crystals are in the form of sand and also called as sand crystals.

Calcium oxalate crystals : In members of family solanaceae. They are found as cubics, rods and prisms.

(b) Calcium carbonate : It is deposited in the form of crystalline masses hanging from a cellulose stalk in
enlarged epidermal cells of leaves of Ficus elastica (Indian rubber plant) and is called as cystolith.

(vii) Latex : It is an emulsion in water having many substances either in suspension or in true solution. It may
contain sugars, alkaloids and oils. It is watery in banana, milky white in Euphorbia, yellow or orange red in opium
(poppy) is dried latex.

Chapter 4 49

Chapter 4 Cell- The Unit of Life Part 4

(viii) Organic acids : Tartaric acid in tamarind, and grapes, citric acid in lemon, orange etc. malic acid in
apple and Bryophyllum. Oxalic acid in the form of crystals.

(ix) Gums : It is formed by decomposition of cellulose cell wall. Gum arabic of commerce is obtained from
Acacia senegal.

(4) Secretory products : The chief secretion of plants are enzymes nectar, colouring matter, water etc. These
secretion are helpful to plants.

Nucleus .

(1) Definition : (Karyon = Nucleus) The nucleus also called director of the cell. It is the most important part
of the cell which directs and controls all the cellular function.

(2) Discovery : The nucleus was first observed by Robert Brown (1831). Nucleus plays determinative (in
heredity) role in cell and organism, that was experimentally demonstrated by Hammerling (1934) by conducting
surgical experiments with green marine unicelled algae Acetabularia.

(3) Occurence : A true nucleus with definite nuclear membrane and linear chromosome, is present in all the
eukaryotes except mature mammalian RBCs, sieve tube cell of phloem, tracheids and vessels of xylem. The
prokaryotes have an incipient nucleus, called nucleoid or prokaryon or genophore or false nucleus or bacterial

(4) Number : Usually there is a single nucleus per cell i.e. mononucleate condition, e.g. Acetabularia.

(i) Anucleate (without nucleus) : RBCs of mammals, phloem sieve tube, trachids and vessels of xylam.

(ii) Binucleate : e.g. Ciliate, Protozoans like Paramoecium.

(iii) Polynucleate : e.g. fungal hyphae of Rhizopus, Vaucheria. Polynucleate condition may be because of
fusion of a number of cells. i.e. syncytium, coconut endosperm or by free nuclear divisions without cytokinesis i.e.

(5) Shape : It varies widely, generally spherical e.g. cuboidal germ cells, oval e.g. columnar cells of intestine,
bean shaped in paramoecium, horse-shoe shaped in vorticella, bilobed, e.g. WBCs (acidophils), 3 lobed e.g.
basophil, multilobed e.g. neutrophils, long and beaded form (moniliform) e.g. stentor and branched in silk spinning
cells of platy phalyx insect larva.

(6) Size : The size of nucleus is variable i.e. 5 – 30µ. In metabolically active cells size of the nucleus is larger
than metabolically inactive cells. The size depends upon metabolic activity of the cells. It is directly proportional to
number of chromosomes.

(7) Chemical composition of nucleus

Proteins = 80% (65% acidic, neutral and enzymatic proteins; 15% basic proteins-histones)

DNA = 12%

RNA = 5%

Lipids = 3%

Enzymes like polymerases are abundantly present and help in synthesis of DNA and RNA. Minerals like
Ca 2+ , Mg 2+ , Na+ , and K + are present in traces.

Chapter 4 50

Chapter 4 Cell- The Unit of Life Part 4

(8) Ultrastructure : The nucleus is composed of following structure

(i) The nuclear membrane

(ii) The nucleous.

(iii) The nuclear sap or nucleoplasm.

(iv) The chromatin fibres.

The nuclear membrane or karyotheca

(i) Definition : It is defined as a regulatory envelope which controls the nucleo-cytoplasmic interacitons and

exchange of materials. Euchromatin Nuclear pore
Ribosomes Parinuclear space
(ii) Discovery : Nuclear membrane, also called
nuclear envelope or nucleolemma or karyotheca, was

first discovered by Erclab (1845). Heterochromatin

(iii) Structure : It is a bilayered envelope. Each Perinucleolar
membrane is about 90 Å thick lipoproteinous and chromatin
trilaminar. Outer membrane, called ectokaryotheca, is
Nuclear envelope

studded with ribosomes on its cytoplasmic surface and Inner membrane
is continuos with RER at some points. Inner membrane,
called endokaryotheca, is without ribosomes and is Endoplasmic Outer membrane
internally lined by electron-dense material of protein reticulum

Fig : Electron microscopic structure of nucleus

fibres called fibrous or nuclear lamina nuclear cortex or hoeny comb layer (about 300 Å thick). Two membranes are

separated by a fluid-filled intermembranous perinuclear space (about 100-300Å). Nuclear membrane contains

following structure.

(a) Nuclear pore : Nuclear membrane is porous and has 1,000-10,000 octagonal nuclear pores. Each

nuclear pore is about 400-1,000 Å in diameter (average size is 800 Å). The number and size of the nuclear pores

depend upon the needs of the cell.

Nuclear pores are interspaced at about Outer nuclear membrane Pore complex Ribosomes Perinuclear space
1000-1500 Å. Each nuclear pore is fitted

with a cylindrical structure, called

annulus (with a lumen of 500 Å) and

both collectively form the pore complex

or pore basket. Annulus has 8 micro-

cylinders (each about 200 Å in diameter

and with a lumen of 50 Å) in its wall. It Inner nuclear membrane Annulus Fibrous lamina
also encloses a channel having
nucleoplasmin for the movement of Fig : V.S. of nuclear envelope showing nuclear pore, Ribosomes and fibrous lamina

substances. Annulus acts as a diphragm and regulates the size of the nuclear pore.

(b) Nuclear blebbing : The nuclear envelope shows evagination. As a result, blebs are formed which are
pinched off. This phenomenon is called blebbing. The nuclear vesicles so formed are thought to give rise to
mitochondria, plastids, etc. Blebbing may also occur from the outer unit membrane only. A row of these blebs move

Chapter 4 51

Chapter 4 Cell- The Unit of Life Part 4

towards the periphery. As a result of deposition of matrix material in between these blebs, and annulate lamella is
formed. The annulate lamellae is thought to give rise to ER cisternae.

(iv) Origin : It is formed by the fusion of ER elements during the telophase of cell division.

(v) Functions

(a) It regulates the nucleo-cytoplasmic interactions.

(b) It allows the passage of inorganic ions and small organic molecules.

(c) It helps in pinocytosis and phagocytosis of large sized molecules .

(d) It allows passage of ribosomal subunits, RNAs and proteins through nuclear pores.

(e) It maintains the shape of the nucleus.

(f) Fibrous lamina strengthens the nuclear envelope. It also helps in dissolution and reformation of nuclear
membrane during cell division.

The nucleolus (Little nucleus)

(i) Discovery : Nucleolus was first observed by Fontana (1781) in the skin cells of an eel. Term ‘nucleous’
was coined by Bowman (1840). Its light microscopic structure was given by Wagner (1840).

(ii) Position : It is generally associated with nucleolar organizer region (NOR) of the nucleolar chromosomes.
It is absent in muscle fibres, RBC, yeast, sperm and prokaryotes.

(iii) Number : Generally, a diploid cell is with two nucleoli but there are five nucleoli in somatic cell of man
and about 1000 nucleoli in the oocytes of Xenopus.

(iv) Structure : (De Robertis et. al 1971). A nucleolus is Perinucleolar
distinguishable into following regions :- chromatin
(a) Chromatin : The nucleolus is surrounded by perinucleolar chromatin
chromatin. Heterochromatic intrunsions are also seen in the nucleolus Matrix
which constitutes the intranucleolar chromatin. Fibrils

(b) Pars fibrosa : Fibrils of 80 – 100 Å size form a part of the nucleolus. Granules

(c) Pars granulosa : Granules of 150 – 200 Å diameter constitute Fig : Ultrastructure of a Nucleolus
the granular part of the nucleolus. They appear like vesicle with a light
central core. The granules may be joined by filament forming a beaded
primary nucleolonema. The fibrils may also be associated to it. The
primary nucleolonema may further coil to form the secondary nucleolonema.

(d) Pars amorpha : The granules and the fibrils lie dispered in an amorphous proteinaceous matrix.
Nucleolus contains large amount of proteins mainly phosphoproteins. There are no histones proteins. RNA
methylase, an enzyme that transfers methyl groups to the RNA bases has been localized in nucleolus. Nucleolus is
stained by “pyronine”. It is not bounded by any limiting membrane. Fibrillar region of nucleolus is called secondary
constriction or nucleolar organising region (NOR) and this region directs the synthesis of rRNA. Ribosomes are
assembled here as such it is also called ribosome producing machine or factory. Ribosomal units so formed are
joined together by thin filament (rRNA) forming a structure like string of beads and it is called “nucleonema”.

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Chapter 4 Cell- The Unit of Life Part 4

(v) Chemical composition : Nucleolus is mainly formed of RNA and non histone acidic proteins. It is a
storehouse of RNA.

(vi) Origin : A nucleolus is formed at specific sites, called the nucleolar organizers, present on certain
chromosomes region (NOR).

(vii) Functions

(a) It is seat of biogenesis of rRNA and also stores rRNA.

(b) It plays important role in spindle formation during cell division.

(c) It receives the ribosomal proteins from the cytoplasm, combines the rRNAs and ribosomal proteins to form
ribosomal subunits.

Nucleoplasm : It is also called karyolymph. It is transparent, homogenous, semifluid, colloidal, ground
substance present inside the nuclear membrane in which nuclear chromatin and nucleoli are embedded. Chemically
it contains. Nucleoplasm is also known as protoplasm of nucleus.

(i) Nucleic acid : Monomer nucleotides of DNA and RNA

(ii) Proteins : Basic proteins (nuclear protamines and nucleohistones and acidic proteins (non-histone)

(iii) Enzymes : DNA polymerase, RNA polymerase, NAD synthetase, nucleoside triphosphatase, and pyruvic
acid kinase, etc.

(iv) Minerals : Phosphorus, potassium, sodium, calcium, magnesium, etc.

(v) Ribonucleoproteins : Contain perichromatin granules and interchromatin granules. Histone proteins are
basic because they contain arginine in much amount e.g. H1, H 2 A, H 2 B, H3 and H4 .

The nucleoplasm helps in maintaining the shape of nucleus formation of spindle protein of NAD, ATP, DNA,
RNAs and ribosomal subunits. Plasmosome and karyosome combindly called “amphinucleoli”.

Chromatin fibres /Nuclear chromatin : The nucleoplasm contains many thread like, coiled and much
elongated structures which take readily the basic stains such as “basic fuschin”. These thread like structures are
known as chromatin fibre. They are uniformly distributed in the nucleoplasm. They are observed only in the
“interphase stage”. Chromatin fibres are made of chromosomes. In resting nondividing eukaryotic cells the genome
is nucleoprotein complex and it is called chromatin.

Chromosome .

(1) Definition : During interphase, chromatin threads are present in the form of a network called chromatin
reticulum. At the time of cell division, these thread like structures of chromatin become visible as independent
structures, called chromosomes.

(2) Structure of chromosome : Each chromosome consists of two coiled filaments throughout its length
called chromonemata by Vejdovsky. These have bead like structures called chromomeres which bear genes.
Chromatid is a half chromosome or daughter chromosome. The two chromatids are connected at the centromere or
primary constriction. Primary constriction (centromere) and secondary constriction gives rise to satellite. The
secondary constriction consists of genes which code for ribosomal RNA and nucleolus hence it is called as
“nucleolar organizer region”. Chromosomes having satellite are called SAT chromosomes.

Chapter 4 53

Chapter 4 Cell- The Unit of Life Part 4

The ends of chromosomes are called Telomeres
“telomeres” (which do not unite with any other
structure). A tertiary constriction is also present in Chromonemata Chromonema
chromosomes, which perhaps helps in recognition Telameresa
of chromosomes.
Centromers Chromonemata
In 1928 Emile Heitz developed a technique
for stainning of chromosomes. Chromosomes can Chromomeres Satellite
be stainned with acetocarmine or fuelgen (basic Centromere
fuschin) there are two types of regions are seen :– Nucleolar

(i) Heterochromatin : It is formed of thick Nucleolus B C D E
regions which are more darkly stained than others A
areas. It is with condensed DNA which is
Fig : Chromosomes A. Diagrammatic B, C, D, E-Different
parts of chromosome

transcriptionally inactive and late replicating. It

generally lies near the nuclear lamina. Heterochromatin are of two types : –

(a) Facultative heterochromatin : Temporarily inactivated chromatin and forms 2.5% of the genome.

(b) Constitutive hetrochromatin : Permanently inactivated chromatin and generally ground near

(ii) Euchromatin : It is true chromatin and is formed of thin, less darkly stained areas. It is with loose DNA
which is transcriptionally active and early replicating.

(3) Chemical chomposition : DNA - 40%. Histone – 50%. Other (acid) Proteins – 8.5%. RNA – 1.5%.
Traces of lipids, Ca, Mg and Fe. Histone are low molecular weight basic proteins which occur alongwith DNA in
1 : 1 ratio. Nonhistone chromosomal or NHC proteins are of three types– structural, enzymatic and regulatory.
Structural NHC proteins form the core or axis of the chromosome. They are also called scaffold proteins. Enzymatic
proteins form enzymes for chemical transformation, e.g., phosphates, RNA polymerase, DNA polymerase.
Regulatory proteins control gene expression. HMG (high mobility group) proteins get linked to histones for releasing
DNA to express itself.

(4) Ultrastructure and Models of chromosomes : (See in genetics).

Important Tips

• Syncytium is multinucleate condition formed by the fusion of cells e.g. in plasmodium of slime moulds.
• Coenocytic is multinucleate condition by repeated Karyokinesis but not followed by cytokinesis e.g. in vaucheria, rhizopus.
• Callan and Tamlin (1950) first to observe nuclear pore in nuclear membrane.
• Staining property of chromosomes is called as heteropycnosis.
• Satellite is also called trabant.
• Centromere or kinetochore is responsible for chromosomal movement during cell division.

• Idiogram : Karyotype of a species is represented with the help of a diagram called idiogram.
• Genome : It is defined as the haploid set of chromosomes.
• Plasmon : Genes present in cytoplasm are called “plasmons”
• Non histone proteins (acidic proteins) are rich in nucleus and less chromosome.

Chapter 4 54


Micromolecules .

(1) Definition : These are molecules of low molecular weight and have higher solubility. These include
minerals, water, amino acid, sugars and nucleotides. All molecules or chemicals functional in life activity are called

(2) Elements : They are naturally occuring and they are classified on the basis of their property into metals
and non-metals. Again on the basis of presence and requirement in plants and animals, they are grouped into major
and minor bioelements. Which are required in large amount are major bioelements e.g. Ca, P, Na, Mg, S, K, N,
etc., while those are required in small amount are called minor bioelements e.g. Fe, Cu, Co, Mn, Mo, Zn, I, etc.

On the basis of function, they may be of following types :–

(i) Framework elements : Carbon, oxygen and hydrogen.

(ii) Protoplasmic elements : Protein, nucleic acid, lipids, chlorophyll, enzymes, etc.

(iii) Balancing elements : Ca, Mg and K. counteract the toxic effect of other minerals by ion-balancing.
There are 17 essential elements in plants and 24 in animals. 14 elements are non-essential :–

(iv) Proportion of elements in a cell

Oxygen - O - 62% Chlorine- Cl 0.16%
Carbon - C- 20% major elements (95%) Sulphur - S 0.14%
Hydrogen- H- 10% Potassium- K 0.11%
Trace elements- 0.75% minor elements (4.25%) Sodium - Na 0.10%
Calcium – Ca- 2.5% Magnesium – Mg 0.07%
Phosphorous- P- 1.14% Iodine- I 0.14%
Iron - Fe 0.10%

(3) Biological compounds : These involve two kinds of compounds.

(i) Inorganic compounds : Characterised by absence of carbon, simple structure with low molecular weights
e.g. water, minerals, ions and gases etc. Water 80% and inorganic salts 1-3%.

(ii) Organic compounds : Characterised by presence of carbon bonded to form a straight chain or ring

Carbohydrates Lipids Proteins Nucleotides Other compounds
1.0% 3.5% 12.0% 2.0% 0.5%

(4) Cellular pool : Aggregated and interlinked various kinds of biomolecules in a living system. So cell is
called cellular pool. It includes over 5000 chemicals. Inorganic chemicals are present mostly in aqueous phase while
organic in both. The aqueous phase may be moleculer solution in which dissolved particles are smaller than
0.000001 mm and colloidal phase in which particle size varies between 0.0001 – 0.000001 mm. Cellular pool
comprises of both crystelloid and colloidal particles. Hence called as crystal colloids the non-aqueous phase
comprises of organic molecules present in cell compartments like plasma membrane, mitochondria, chloroplast, etc.

Chapter 5 55

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